A geared up Shaman is pretty lethal, in one of the horde main pvp groups we go up against, they have one of their aces in the hole - Solidus - running around, the guy is geared up - weapons and armor - beyond belief and has put some devastating crits on me, I got some kind of weird shock lightingish thing (whatevertf it is they do) one day while trying to save a flag cap in Arathi Basin, and just before I got to interrupt, I got a crit on me from him for about 4750. My max Health is like 3800, so yea, ouch.
Priests are also pretty sick in pvp synth, but I don't think you're looking forward to speccing out of holy and getting your guild a lil miffed at ya
Rogues and hunters are pretty fun at pvp (especially dagger rogues..I'm a sword rogue though, it's still pretty fun, might switch back to daggers after I make grand marshal..2 weeks I'll be making marshal hopefully and will start having to make top 10 or top 5 standing.)
But just about any class can pvp well with the right spec and half decent gear (don't need superb epics to kill, but they sure do help rack up the numbers =P )
Of course though, if you play a rogue you'll really really learn to hate hunters...pretty ghey when you get a hunters mark thrown on you from halfway across the freaking continent >_> ) Also..bleh..orc hunters SUCK >_<