Actually druid mitigation is pretty impressive, they have very high armor pools, savage defense; which mitigates damage similar to shield block but which is based off crit and ap, they also have talents that further reduce the damage they take, a talent that makes them crit immune regardless of defense, as well as a talent that enables them to self heal off of their crits. In addition to this they have monstrous health pools, and a couple of decent cooldowns as well.
Warriors are always going to take spikier damage than druids or pallies because our mitigation is circumstantial. We don't have holy shield to smooth out the damage, we don't have ardent defender to provide the cushion where it is most needed (near death) and we don't have the extremely high armor pools of dk's and druids to flatten out all incoming damage. We have the ability to double our shield block value on a cooldown, and a chance to proc a critical block to up our block as well, but this is subject to the rng.
I personally enchant for defense as needed to hit 540ish, then stam on the rest of the pieces, gem for stam (unless i feel a socket bonus on armor is worth shooting for with some kind of purple/green/orange gem) and play with my trinkets a bit as needed. I imagine that is a fairly prevalent view among most of us (other than some folks who will simply gem for stam regardless of socket bonuses and trinket for stam no matter what).
Armor is great but in reality far more governed by item level than any other consideration and since as a general rule we tend to gravitate towards the higher level pieces anyhow, armor tends to take care of itself. There are always pieces that prove the exception to the rule but overall stam stacking tends to be the highest priority in gearing after hitting 540 defense no matter what kind of tank you are.
I know nothing of Dks, sorry.
For my money though, pallies are the top of the tanking tree, followed closely by druids. Warriors are the most engaging though

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Warrior 4 life.
(the druid is an alt, I swear)