Gonna start with my least favorites first because my hatred of things is more memorable than my favorites, which i'll edit in later.
least favorite
10: Skill link. Pokemon Reborn's Noel has made me hate this ability, especially with Cincinno. Any fast mon with multi-hit can go full choo choo mode. When you pair it with a Razor Fang, it turns the abuser into a very scarey flinch-haxxing machine.
9: Pixelate. fuck fairy types. nuff said.
8: Mummy. listen here you little shit. Switching a mummy into a physical attack is a pretty good way to say fuck you to a lot of things. It ruins my generally very picky physical playstyles as soon as I am prepared to destroy.
7. Imposter. You better check yourself before your wreck yourself, because any ditto that comes in on a stat buff will instantly be your demise.
6. Stance Change: Fuck aegislash
5. Gale Wings, see above post
4. Protean: fuck greninja
3. Serene Grace: flinch paralyzed flinch flinch paralyzed flinch flinch flinch flinch flinch paralyzed flinch flinch flinch paralyzed flinch flinch flinch flinch paralyzed paralyzed flinch flinch flinch flinchflinch flinch flinch flinchflinch flinch flinch flinchflinch flinch flinch flinch
this mechanic is fair and balanced.
2: Unaware. Fuck you. Fuck your shit. get out of this game. (i spend a good amount of time setting up)
1: Prankster. I have a love-hate relationship with this ability. I love it because it expands so many options in battle, but is easily face-turned-heel, and it's hell to deal with. Ame's gen 5 team still gives me nightmares.