I'm currently leveling up a paladin and a warlock together. I won't claim that they kill things the fastest (probably double mages for that) but it does have the benefit of zero downtime. Paladin tanks while Warlock has Salvation up. Warlock dots everything up and then watches them die. Lifetap for mana whenever health is full and paladin can heal up the warlock as needed. And you can put Soulstone on the paladin so you never spend any time having to run back to your body if either die.

I'm planning to AOE grind later on, paladin tanks while Warlock kills everything but my guys aren't quite high enough for that yet (and I'm not quite skilled enough at controlling all their abilities at the same time yet, only started this week).