As far as I'm aware, here's how things stack:

Most Threat: Dark Knight, in both avenues.
Highest Physical Mitigation: Paladin
Highest Magical Mitigation: Dark Knight
Highest Burst Mitigation: Warrior
Single Target DPS: Warrior > Dark Knight > Paladin
AOE DPS: Definitely not Paladin, but I'm not sure otherwise.
Utility: Paladin has the best defensive utility, Warrior is a great middle ground between offensive and defensive, and Dark Knight is lagging behind both in this category.

Duo: I doubt that any raid group is going to leave home without a Warrior, with the preferred co-tank depending on the fight itself. Paladin's always a safe bet, but a Dark Knight is right at home when fights are on farm or if magic damage is prevalent.