For me (no particular order)
Monks - I can't kill the healers, which is the worst 1vs1 arena battle in the history of battles. Forget them running low on mana, their mobility and cc abilities will either run down time, or outlast you. The DPS monks are also just as annoying - disarms, silences, slows, stuns, immoblize, Touch of Karma - ridiculous class that just pisses me off to no end. I make it my business to personally help kill each and everyone I see.
Warlocks - Fear spamming that never breaks on reasonable damage, stuns, and a stupid pet that puts you to sleep mid burst or follows you around and tried to carve you up. Don't get me started on the hell spawn burst when my screen explodes with a bunch of imps crawl all over the place. And, if you manage to survive all of this, you'll have to contend with a bubble and heals as good as our own.
Mages - Polymorph -----> Deep Freeze. Not as annoying as the used to be, but still can be pretty tough, or down right impossible when played correctly.
Shammans - WTF. I find myself screaming at my screen when my health goes from 60-0 in once GCD. And I'm still checking my logs to figure out wtf happened. Seriously...what happened?