Damn hard question, in my opinion this:
1. In battlegrounds
Unholy DK, Shadow Priest, Affliction Warlock, MM hunter, Ret
2. in 1v1
Unholy DK, Ret, MM Hunter, Spriest, Destro Warlock
3. in Arena
War(but with bis healer), DK(also with healer or retrymourne), Shadow priest(free dot spammer , or mind flay but u have to be god dispeller if u wanna get high), MM hunt(all instant all doing 4k atleast + 100 deffensive and tons of cc spells), Destro warlock(1 destro chain and can nuke everyone but hey..)
Hard classes IMO:
Sub Rogue
Fire/Arcane/Frost Mage
Protection paly
Feral
Disco Priest
Resto Shaman
Destro Warlock
Elemental Shaman