Death Knight would be the best, IMO. Too much you can do to lock down any given class. Warriors and Rogues sometimes have a chance if they play VERY well with equal gear. I spend a lot of time dueling in order to feel out the classes I don't know, and It has gotten to the point where I spam trade offering 500g to anyone able to beat me in order to get people to try.
Pros
Lichborne fear immune/DC Heals
AMS vs Casters/Fear/Misc CC Also gibbs Paladins and fuels DC spam.
IBF Stun immune Ghoul stun a rogue on their open, pop IBF before Kidney Shot.
Strangulate to zone in on casters in order to save DG, also good to gibb paladins and other DKs before they get momentum.
Ghoul Sac
Ghoul Stun
Gargoyle's passive pressure
AOE Slows
Chain CC with no DR
Much more..
Cons
Cannot fly while in combat.
P.S. Loldeathstrikespam