Greninja with Protean and Aegislash. After trying to battle on Pokemon Showdown several times, I quickly discovered that I hate battling these two Pokemon. Also, Garchomp, Tyrannitar, etc. Basically, any of those super powerful Pokemon that are Overused. I like seeing variety and just get irritated by them.
In-game, anything that can inflict status effects. Particularly those with abilities that inflict it upon contact. It feels like it gets inflicted SO often and the RNG is stacked against me a lot of the time.
Also, Steel-types. They're just so ungodly defensive and the types I like using are often walled by them. (Psychic, Fairy, Grass, etc.) Luckily, I also like using Fire-types, so there is that. But still, I think they need a bit more of a Defensive nerf in my opinion.
Ground-types because of Earthquake hitting my favorite types for super-effective damage (Electric, Fire). Plus, Sandstorm. I really really really hate Sandstorm.
Dragon-types because it's really difficult to score a super-effective on them, even with the addition of Fairies, and they block all of the primary elemental types that I like to compose my teams of. They still feel kind of cheap even in this day and age.
Things with Sturdy. I think it's been taken down, but nope! Still alive! Whilst I can get rid of it the next turn easily, it's still a pointless time waster in-game and a huge annoyance.