The single absolute worst thing that an author can do in a crossover where a character ends up in another world, is to also send their villains or enemies to that world for a "villain team-up." I don't know where this absurd, frankly stupid notion comes from that it's a good idea or that the scales need balancing or something when that defeats the whole purpose of seeing how one and only one additional character would change the course of events.
One of these days I'm going to put up a prompt or request, or single chapter "continue the story from here" for a Persona crossover with something or other.
I enjoy reading:
- Persona, as you can probably tell from my name. I particularly like stories that include Aigis, Naoto, or a Female wild card holder as the main character. Just about any character focus can be good, though.
- Worm, just about anything well written using Taylor's perspective, but I'd prefer it actually take time for character building and give the characters breaks instead of skipping from crisis to crisis and focusing on constant action like the serial seemed to do. Bonus points for things involving Amelia, Paige, or the Simurgh.
- RWBY, it's just a great setting with several good characters, and as a bonus it's easy to pull characters out of or put others into due to the relatively low power cap.
- Miraculous: Tales of Ladybug & Cat Noir - The title characters of course, but I could see other options working. It's also pretty much the only thing I'll bother with one-shots of.
- Harry Potter, especially centered on Luna or Fem!Harry (or both), and/or making use of time travel, very AU, or alternative background living conditions. Kind of an exception to the general hatred of time travel.
- Firefly, centered on River, especially crossovers.
- Log Horizon, especially Akatsuki, and Sword Art Online. The latter has seen some great P3 crossovers, too.
- Pokemon, particularly if ghost types or any of the Incredibly Adorable Three (Vulpix, Ralts, Petilil) are involved. By contrast, I hate the anime; Ash is an idiot, Pikachu is lame, and if you're going to have a massive criminal empire, don't make them totally incompetent.
- Stargate, almost always as part of a crossover.
Crossovers: Some universes and characters work well as a 'source,' and others work well as a 'destination.' A few like RWBY work as both, but for the most part, it's best to make certain characters get nudged to certain settings, but not the other way around. Likewise, crossovers work best when one, and only one character gets moved to a new setting, and cannot return for the duration of the story. Introducing additional elements from the 'source' universe almost always makes a story worse, not better. Again there are a few exceptions where you might take two, or even three characters away from home, but this is not typical. Just going from the list above as examples...
- Persona makes a better source than destination, but you can get away with either one. Worm makes an excellent destination (as the plethora of alt power fics will tell), and a good but sadly under-used source. Rosario Vampire makes a fine destination, but a terrible source - never pull characters from it to go somewhere else, it just doesn't work. Harry Potter makes a good source, but one of the worst possible destinations - sending characters from other worlds to Hogwarts is the premise of many fics, and it's stupid. It doesn't work. Don't do it, ever. Firefly makes a good destination, but not much of a source - the characters are pretty rooted in the setting and wouldn't adapt well to others. Log Horizon makes a good destination (as do Sword Art Online, Overlord, pretty much anything involving an MMO), and these types are easy to get characters into, to boot. Kinda pointless to use as sources, since they're MMO characters, except between each other. Pokemon makes a good destination, but not source. Other characters suddenly in the pokemon world? Great. Pokemon suddenly outside of their own setting? Meh, the main problem is that puts a huge limit on the whole point of pokemon, though Worm can manage it sort of with the right power. Stargate makes a good destination, but just doesn't have any characters that are that interesting outside their setting, so is never really a source.
- Crossover fics are invariably better if the worlds are totally separate. Making them parts of the same world, where one could travel between them freely, removes a lot of what makes these fics good, and opens up way too much potential for things that make them worthless (such as adding in more characters from Source-world). Stick with alternate universes where possible (which is always, unless you're doing one of those HP/Stargate things again).
- M/M Slash. Writers have a bad habit of making it too explicit, and it's just really not my thing. Also, I just plain can't identify with it.
- Highly descriptive smut. Please just fade to black, that stuff is outright boring to try to read through. Same with long battles, combat is one of the most boring things to read...
- Authors getting known character names and relative ages wrong. If you deliberately change a name, say so before the first chapter, and likewise if you're going to change how old someone is.
- Authors getting basic details about the setting wrong, ones anyone would know and easily remember years after reading or seeing the source material even just once. Also major things that are easy to look up, for that matter. If you don't know highly common details about the universe, don't write in it; if you're writing, get things right. AU is fine and all, but if you don't note that major world fundamentals are going to be changed, it just comes across as incompetence.
I keep my favorite lists short, since 'favorite' means something. If someone is on my favorite authors list, it means they've both completed more than one story that I found favorite-worthy, and I seriously like either a majority (more than half) of their stories that are 40k words, or (if they've written way too many in fandoms I don't read) 90% of their stories that I have read. Favorite author is serious business, you don't assign such a strong word to just anyone. Granted, there are probably a few people who would be on that list if they'd written more than one or two stories, too...