Anything in particular that my team is weak to at the time, but that's mainly in the beginning of the game (ie. finding Geodude when all I have is Cyndaquil). Overall, I really hate finding Boldore or anything else with Sturdy. I can't think of anything I really hate from gen I-IV though.
Edit: Oh! Protect/Detect spammers! Hate those.
Edit: Oh! Protect/Detect spammers! Hate those.