I heard Affliction Warlock with Epidemic - Richie's Affliction Warlock PvP is supposed to be amazing for Battlegrounds/RBGs. So much so that I am levelling a warlock just because of this.
depends on what you personally like i would say...
if you like to kill a lot of people any class with a kill shot (eg warri / hunter) is nice.
If you want to be a bad ass i would choose a stealth class like rogue
I personally find hunters very funny, but on rbg i enjoyed the rogue or warri more because of the bigger impact you have to your teams success
While talking about impact on team success we should not forget the healers... so, how about a healer?
Im playing Affli warlock with Gladiator Suite. 5 matches today at 2k rating and was top dps in all of them. GS routines are best routines i have played with (in pvp).