After reading through the actual listing of
fanfic tropes, I've found more that I'd forgotten or didn't know the name of that I despise the majority of the time they are used. Most bad tropes can be okay if used well, though. Doesn't mean it ever is, but I'll admit the possibility exists.
My top 5 from least disliked to most disliked:
5. Modern AU Fic: Fics set in a contemporary (and often mundane) setting. (Rarely seen one of these that couldn't have been better with an original character. Especially when they change/remove/delete the characters original skills/powers/memories/etc. Just make an OC at that point.)
4. High School AU: An AU fanfic in which the characters are high school students. (Never seen one of these that was worth reading in the first place. Usually the characters are from some setting that doesn't have high schools and the characters don't have their canon skills or memories. So essentially reskinned OC's with a personality overlay.)
3. Het Is Ew: Boys kissing girls? Disgusting! (Most often seems to be someone's yaio or yuri fetish being forced on everyone in a fic rather than written well on a few characters.)
2. Omegaverse: A setting where humans have a secondary sex (alpha, beta or omega). (99.9% of the time it's someone's attempt to justify NTR.)
1. Mister Seahorse (MPreg): A male character gets pregnant. (I despise these so much that I try to forget they exist. I like to think I have a very good ability to suspend my disbelief, but never enough to read one of these with it redlining.)