I got into fanfiction in late 2013 when a friend recommended Harry Potter and the Methods of Rationality to me, and I haven't looked back since. I started out deeply entrenched in the Harry Potter fandom, but left it when I felt it was almost entirely stagnated - nowadays, I mostly read RWBY, XCOM and WH40K fiction.

Most of my favorites are pretty epic in scope and also tend to be very AU - but every one of them is an experience. You won't be disappointed by any of them - I guarantee it.

My most hated trope in fanfics is probably time travel - the foreknowledge generally ends up making the plot boring as hell, and there's not much the author can do to avoid that. So don't do it, people - it's lazy! Go write an AU or something, don't just rehash canon with your garbage fix-fic!

More dubious advice for aspiring authors purely from my observations about things I like -
1. If you want to make a fic with the biggest possibility of success, explore somewhere that canon didn't. Go somewhere else in the world! View it through the eyes of a supporting character! Go to the past or the future, or a vastly alternate present! Make OC's! Well... Make believable OC's that add to the story instead of taking it over! If you make an SI, which you probably shouldn't, make them flawed! You sure as hell aren't perfect!
2. if you only have one protagonist, focus on a good number of other characters as well - It's hard to believe that anything lastingly bad will happen to a single character, but if you have a group, you can kill 'em off to keep your readers guessing. Also, a diverse cast can redeem your story if your protagonist sucks.
3. You don't need to put every character in a relationship with another. Not only is it unrealistic, it's also hard to write effectively.
4. Avoid harems unless that's the main focus of the story. You end up having to either spend too little time on each prospective love interest, or spend so much time on them that it really gets in the way of the plot.
5. Get someone to beta for you who isn't afraid to call you on your bullshit. It will improve the quality of your writing greatly, as well as reminding you that only you enjoy reading your pet OC. Did you know that one of the best ways to convince a prospective reader to keep reading your fic is to not have any glaring grammatical errors? Sad but true!

Useful rules about power levels:
The basic one is if you power up an existing character, you had better make their conflicts matter still - either by making their enemies more powerful, or by focusing on internal struggles and character development rather than physical conflict.
For Crossovers: The best crossovers tend to be those in which the relative power levels are similar. If you stuck Goku at Hogwarts, that shit would be a joke - but a lot of other magic settings work just fine. If the power levels aren't similar, you just have to approach it differently. You could make an AU in which the crossover elements have reduced power, or you can follow the above rule. Just remember - curbstomps are fun for like five minutes, which is when you realize that there will be no meaningful conflict in the story. Please don't do that.

When I read stuff on the internet, sometimes it's not based on existing works - it's original fiction!

WORM is a gritty superhero fiction that deconstructs the problems a world with superheroes would face, and it's filled with characters that are fun to read - because they are actually smart. It's also got a pretty massive fan following, with many stories posted here, and far more at Spacebattles and Sufficient Velocity (literally hundreds). It is the breakout creation of esteemed web serial writer wildbow, who has also written some other stuff that is equally original and magnificent. Check it out, but be prepared to lose like a month of your life to it. It's LONG.

UNSONG is a pithy fiction about an alternate reality where the things that exist in the Bible and other Judeo-Christian religious texts are a bit more than metaphorical - and have staggering effects for the world as we know it. It's ridiculous and fun, and it manages to achieve a very different feel from most other fics you would find. Although I guess it's technically a fanfic of the Bible? Whatever.

When I'm not digging for diamonds in the mountains of dirt on this site, I like to find the 0.1% of smut fics that are well written. I'd never post them on my profile, though... I have standards! (jk im a worthless degenerate)(also pm me ill find some for you)

lol too bad no one will ever read any of this because i have never published anything