You'll need GLD leveled at least a bit no matter which tank you decide on, so perhaps start there. I personally found WAR to be more fun and easier to hold aggro, at least at lower levels when they're very nearly the same class except MRD has overpower (which is great, especially when learning before 30 when you can also have invigorate). I haven't played DRK, though I imagine it's a bit closer to WAR play style than PLD.