It depends on how the fight is paced, how many hits the tank busters are, whether there's cleaves or not, etc. but basically Sheltron destroys that argument for any fight that isn't magic based (so, unfortunately, just A3S, Thordan EX, and Ravana EX atm). Not only is it ~29% damage reduction, it can be stacked with other CDs - obviously not additively but I think the only equivalent of Sheltron + Sentinel is DA + DM + Shadow Wall + Reprisal, and I'm not sure if that even adds up to enough. On top of this, an advantage of both DRK and PLD is that their short-CD-high-damage-reduction abilities are available out of their tank stances, unlike WAR which needs Defiance for IB. I think WAR might have the most overall mitigation in a given fight if using tank stance, but DRK/PLD have the highest burst mitigation rotation available to them with or without their tank stances. PLD also uniquely has Divine Veil, though that hasn't really needed utilizing yet.