Suppose there's a story where a male character (often "male" with extreme scare quotes) becomes female. What device is used to accomplish this? It could be a magic potion, some sort of lab accident, a realistic depiction of HRT, or many more options. Which ones are best*?
I was going to write this in a gender-neutral way, to be about genderswaps in general, but that wasn't tenable. The particular genders in question affect how it comes across in practice too much. Also, my experience is mainly with the "male-to-female" type. Other people, with different experiences and preferences, are likely to have significantly different rankings.
Other people are also likely to have different ideas of what the genderswap mechanisms to rank even are. This does not divide up neatly into one obvious set of categories.
*Nothing here should be read as a moral endorsement of any of this fiction. All fiction is bad actually.
Makeover: This is a huge category, but a hard one to split up. Changes to clothes, hair, makeup, and so on cover a lot of ground, and can easily be drawn out into many steps. This can draw on real experience more easily than most mechanisms, and is likely to be relatable or easy to imagine oneself in. Very solid mechanism, incredibly flexible. A staple, really. Also, there's plenty of room for having it read differently depending on the motives of the person/people giving the makeover, and how willing the recipient is.
Deals, bets, and dares: Wonderful. Needs to be combined with other methods, as the content of the deal, bet, or dare, but this wrapper adds a lot. Ideally, this plays with consent by making the deal seem worthwhile at first, luring the character into something bigger than they realized. This is a case where making it more complicated actually adds value.
Hypnosis: Powerful, versatile, and just plain fun. Fictional hypnosis can change someone's perception of their own body, mess with their memories, change their behavior, and best of all, make them confabulate elaborate justifications for the changes that keep them from realizing the truth. There's a tremendous amount of potential here, and it easily combines with other mechanisms. Obviously good for setting up complicated stuff with agency and consent.
The cartoonist changed genders so now you have to: Tends to come up in webcomics with self-insert characters, but not limited to that. Usually involves the character talking a lot about their gender feelings, sometimes directly in conversation with an embodiment of their repressed gender or a separate embodiment of the cartoonist. Impact depends heavily on the skill of the writer (which you presumably like if you're reading that comic in the first place) and how well it's justified by past canon. This can be combined with other mechanisms (storyline where the character gets on HRT, for example), or can be purely an instant character redesign by authorial fiat. This should probably be lower objectively, but I'm steeped in old webcomic culture and am therefore a sucker for it.
Purely social (decided by others): You're the same, but people around you decide a new gender for you. This is great: very embarassing, doesn't require a decision on the character's part, has horribly messed up power dynamics, can easily incorporate all sorts of gender signifiers being imposed on the character. Compatible with her realizing she actually is a trans woman, or even having known that beforehand.
Narration: Simply narrate the character as being progressively more feminine. For example, frequently mention their clothes, and each time have it be a little different, gradually going from men's jeans to a miniskirt. There's no scene where they go get new clothes to wear, it's just that what they've been wearing all along changes. Obviously, this can be done for arbitrary characteristics. Ideally it's subtle at first, seeming like minor inconsistencies, but ramps up once the reader can be trusted to have caught on. It's a fun effect, with potential for all sorts of corruption angles. In practice, this is done with at least a hint that it's representing someone in-fiction using some sort of reality warping power. But what if that could be completely removed from consideration? What if it was purely about narrative force? What would that say about the author and the reader?
Hormones: Realistic and relatable! No marks for escapism, but it can be a good choice if you're drawing on your real-life experience for the story. Commits you to a long timescale for the transformation. Giving this to someone without them realizing is an attempt at a nice dynamic but doesn't quite ring true; it's hard not to think about the logistical difficulties. Fun detail that doesn't get enough attention: taking estrogen sublingually involves not being able to talk for a while, which can be a nice little moment of vulnerability.
Lab accident: Generally implies science fiction, perhaps set in the present but with an unlikely invention being tested. Good flexibility, both in terms of the effects and the reversibility: it can easily be a freak accident that can't be replicated or reversed, or maybe the story becomes about this person now having a means for freely swapping anyone's gender. Reasonable mechanism whether you want to go in-depth on the made-up science or not.
Surgery: Potentially realistic, usually presented as more of a big total thing than the various more specific surgeries for real-life trans people. Planning time and recovery time are usually handwaved. The effect is often that it feels mostly plausible while still having increased effectiveness in creative ways. Can be opted into or done to you without your consent; either way, the feeling of irreversibility is good.
Magic spell: A fairly broad category. You can make this work pretty much any way you want. The big question is how much the story becomes about your made-up magic system. When left unexplained, it's a boring imagination-stopper of a mechanism. On the other extreme, genderswaps become mainly an excuse for giving expository speeches about how the magic system works. You might think the right thing to do is take a sensible middle ground, but you'd be wrong. Go all the way towards magic system exposition. It'll be great. Remember, your target audience loves intricate systems.
Curse: Like magic spell, but with the magic system definitively considering it to be a bad thing. This rules out some options, but the opinionated metaphysics gives a little extra weight and something to rail against. This also provides the story with an easy framework for investigating ways to break the curse and encountering obstacles to doing so. Those obstacles can scale up arbitrarily to keep the character female. In extreme cases, the curse becomes a law of physics so unstoppable that even attempting to break it turns anyone else nearby into a woman too.
Clothing: Specifically, clothing that effects some transformation beyond the normal scope of a makeover. "Halloween costume turns out to be magic" is a common subtype. This takes the obvious fact that clothing affects how someone's gender is perceived and intensifies it, which kind of seems unnecessary given how intense that already is, but I suppose that's to address readers' fears that putting on a dress won't do enough. Worse than mundane makeovers due to having insufficient faith in the power of makeovers on their own; still has room for a lot of fun.
Reversal of surprise genderswap backstory: Reveal that the character was already genderswapped, from female to male, and then restore them to their original state. This revelation can be a surprise to the character, and ideally should. The big advantage here is the feeling of inevitability: of course the character is going to be a woman, because she always really was. The two most common ways to implement this are magic and a shaky understanding of intersex conditions. The former is preferable.
Bureaucracy (mundane): There's some error with the gender field on your driver's license, or you were tricked into signing a name change application, or a judge issued a court order that you have to be a girl now. Could be done as an accidental systemic quirk, or as something a hacker did to you. Good inversion of the typical way bureaucracy interacts with transness, by having the frustrating errors and indifferent systems actually enforce a gender change instead of interfering with it.
Genie wish (accidental side effect): This requires finding a way to interpret what was intended as a non-gender wish so that it results in a gender change. If you can pull that off convincingly, it's a satisfying effect. It should reasonably follow from the wish's wording, but in a way the character didn't foresee. Ideally it feels like this is more about them than it is about the genie forcing a weird interpretation. A good bonus here is that it means you've put at least some thought into a desire the character has other than becoming a girl.
Change of context: Not a change in the character himself, but the social context around him. Could be something as minor as going to a different region where what he'd thought was a masculine hairstyle is considered feminine instead, could be full isekai into a world where gender stereotypes/presentation are implausibly exactly reversed. This is sort of a cheat, but it's a creative cheat, and tends to involve interesting thoughts about what gender means.
Food: Ranges from enchanted cake to getting real into the idea of soy boys. A little silly, but the metaphor of gender as something you get by consuming is decent. This is well suited to having effects slowly ramp up over a long period of time, and since eating food is such a normal background part of life, it works for having the character not realize the cause. For "I hid estrogen in your food the way you'd trick a dog into taking pills," see hormones.
Potion: Also covers versions where it's some genderswap chemical a scientist made, because come on, that's still a magic potion. This ranges from instant transformation to a fantasy-setting translation of modern HRT. Fairly direct, doesn't add much in its own right. Can be a reasonable way to implement a genderswap when you don't want the mechanism itself to especially matter. Improved by getting into details of how the potion is brewed, or giving it an unusual set of effects.
Merging with alternate self: There's an alternate timeline version of you who's a woman, and you merge with her or become her or relive her memories in your dreams, or something along those lines. The narrative purpose is to be a thin metaphor for coming to realizations about your true self, and also to satisfy the common trans desire for the gender history that you'd have if you'd been cis. That's not especially a coherent desire when you look at it closely, so the mechanics of it working in fiction generally don't make too much sense, but it's undeniable that this is tapping into something important.
Reprogramming: Roughly similar to hypnosis, but generally requires the character to be a robot or AI or something along those lines, which limits the scope of this mechanism severely. Effectively functions as an unnecessary metaphor for more "realistic" mind control transformations. Can be improved by getting deep into the worldbuilding on what AIs are like, or just by playing up the software engineering angle. Remember, a lot of the people who are into this stuff have programming jobs!
Avatar: Play as a girl in a VR game, or in some telepresence setup, or maybe there's an uploading sort of thing going on. When this is temporary, and your meat body is still around operating as usual, it's pretty incomplete, at most just an impetus for realizing you want to be a girl. There's more weight if this is treated as a full genderswap mechanism, essentially becoming the avatar and forsaking meatspace (perhaps out of necessity rather than by choice). Ultimately this usually doesn't get at the heart of genderswapping directly enough, but it can be a fun element leading into gender exploration stories. Sometimes combined with mind control to interesting ends.
Booth: Go inside, something happens (details often unspecified), come out as a girl. The black-box nature allows flexibility in how much it changes; maybe it does clothes and makeup. Has obvious coming-out-of-the-closet resonance, and accordingly tends to be used intentionally, though being tricked into a genderswap booth is quite possible. Employed to good effect by Powerup Comics.
Coffin: Basically a booth that fell down on its side.
Body swap: High potential, since interactions between the two people being swapped are fun, but so tempting to reset that it usually falls flat. If reversing it is an option, having both the people turn out to be fine staying swapped feels extra contrived, even by the standards of this whole list. I want this to be better than it is in practice. Ending with everyone going back to normal but with increased understanding of each other isn't pushing things far enough.
Genie wish (intentional): Too easy. Tacking on a monkey's-paw twist where there's something horrible about the specific woman they become can be fun but feels spiteful in an arbitrary way; this can be improved through character development for the wish granter (to explain why they'd twist it that particular way) or for the genderswapee (to make the twist feel like an appropriate comeuppance).
Purely social (self-directed): Asserting your gender is different now, taking no other action toward this, and having it stick. Admirable level of force of will, rare to see this without at least some changes in behavior/presentation. Mostly this just highlights how unfortunate it is that in real life, it takes more work to actually get consistently gendered the way you want by others.
Touched the wrong crystal: Femininity as a magic crystal is interesting. It's mysterious, passive (you go to the crystal, not vice versa), and alluring. But also hard, sharp-edged, and dangerous. Ultimately this suffers from the fact that the crystal itself doesn't do much to affect how things play out afterward. It could generally be swapped out for other methods without changing the plot.
Medical accident: Similar to the surgery category, but instead of being opted into, it was done when it wasn't supposed to. Maybe you were in there to get a kidney removed and they mixed up their files, maybe you were in an accident and this was the only reconstruction they could manage, maybe you're the time traveler in "'—All You Zombies—'". There's not enough agency behind this. At least have someone deliberately cause the mix-up. Yes, systemic forces can be compelling as a genderswap mechanism, but they have to actually be systemic, and this one always comes across as a weird one-off within the setting.
Weird goo: Often literally pink, always spiritually pink. Popular but kind of silly. It's a pretty clear mechanism: obviously touching weird goo will do something to you. Useful shorthand, doesn't add much beyond that.
Put in body of fictional game character: Distinct from avatar in that you're literally put into their body. For example, Jack Black in the Jumanji movie, or Ash in Die. Good instant surprise value, allows for all sorts of attributes for the new form, but potential for long-term growth is often squandered because leaving the game is such a convenient reset button at the end.
Possession: Lacks the visceral impact of your own body getting genderswapped, raises many questions about the person being possessed. Are they still there as well? If so, what sort of shared control/communication is there, and how do they feel about the situation? There's room for interesting dynamics here, rarely explored enough. If they're possessing you back, this is a body swap instead.
Soda: The implausibility hurts here. Sillier version of potions, and potions are already fairly silly. Ideally you lean into that and give it a ridiculous name, or a flavor that's some forced pun, I guess. Honestly, I'm not sure why this comes up often enough for me to have seen it multiple times.
Gun: There's no way to make this not goofy, so you might as well go with it and make it the wackiest-looking mad scientist ray gun you can come up with. Invokes violence and positions the person being transformed as a victim, but the goofiness detracts from seriously examining this.
Reincarnation/isekai: This is when a character dies, then starts a new life as a woman. The main drawback here is that the discontinuity is very sharp; it's missing out on the fun of dwelling on intermediate steps in a progression. And honestly, the rebirth metaphor is better when it's less literal than this. As with anything isekai, it's an immensely flexible premise whose potential is generally squandered. But I'm not penalizing it very much for isekai being cringe, because this entire list is such a glass house in that regard.
Just woke up as a girl one day: Really just a lack of mechanism. Very boring. Maybe you have good stuff to do with it afterward, but you could just as well take the time to come up with a real mechanism and still do all that good stuff. There's no reason to pick this unless you have a very tight length limit and dislike your audience.
Body suit: You stay the same, but you're wearing some sort of latex suit or mech sort of thing that results in a feminine exterior. Does that mean you're still a guy in there, and it's not much of a genderswap at all? Well, not necessarily; you could get something out of the idea of gender being this literally external. Mostly, though, this is about fetishes that don't do much for me.
Disease: As a mass pandemic where a whole lot of people are getting genderswapped, this is too much global disruption for it to feel meaningfully like a man turning into a woman. The context is just too changed for that to mean the same thing. If it's some rare syndrome you come down with that turns you into a woman, that takes a lot of work to not come across as an unsatisfying black box.
Bureaucracy (celestial): As in Misfile. Typically opens up a lot of questions about how the celestial bureaucracy works and why there's a divine database of genders to begin with. This is pretty overblown, but does have a good feeling of inevitability. Requires lots of worldbuilding work to get me to not be annoyed with the implications. Might someday be done well.
Aliens: It's not easy to have a good reason for aliens to care about changing some man into a woman. It's very easy to have a bad reason, or no reason. Generally just comes across as an unexplained plot device and draws attention to flaws in the worldbuilding.