If you want survivability, I'd go with a Prot-Spec Paladin. You can deal decent damage and take more punishment than any other class in the game. It may not be the fastest, but you can take on 3 mobs and once and not have a problem as long as you heal. You will be tanking in instances, and considering that there are only two other real tanking classes in the game (Druid and Warrior), you won't have trouble finding a spot in raids or instances. You can also switch to Holy if you desire, as Holy Paladins are also a hot commodity end game.
If you want speed and effectiveness, however, I'd say Warlock. I'd go with Affliction and abuse Howl of Terror/Fear while you DoT all the mobs and have your Imp out for the extra health. Use Drain Life to get health back, and Life Tap to get your mana back. Load up on all the stanima gear you can get your hands on and you'll be good to go.
If you feel like it, you can switch to Demonology spec at lvl 50 and use the Felguard. They are the only pets that can really tear through enemies quickly, and combined with your DoTs, you'll plow through mobs while maintaining a safe distance.