I'd say Trick Room and the goodstuffs. Goodstuffs because they're... good. Tyranitar, Metagross, Salamence, CRESSELIA are always hard to counter and beat, especially for those who try to think outside the box and don't use goodstuffs xD
Trick Room because my only way to deal with it is to Taunt the Trick Room user. Once they get it set up, it becomes a pain since my team is all about speed and striking fast.