Easiest class to solo level is Hunter.
Followed by Warlock.
But Hunter suck ball in pvp.
Warlock is the best caster pvp class in TBC.
If i was you and want to pvp at lvl 70 i would roll a Undead Warlock.
Easy to level up, Free lvl 30 mount. No drawback in pvp, Spec SL/SL at 70 and destroy everything but rogue.
its easy to PVP as warlock at 70, Spec SL/SL.
Put Curse of agony,Siphon life, Corruption and Immolate on target, Drain health until nighfall proc, Shadowbolt your ennemy, Watch them die easily.
Its one of the most OP Spec in Wow history.