General consensus is tank order goes:
(1) Warrior
(2) Paladin
(3) Dark Knight
I would guess most people feel the (1) and (2) spot are fairly comparable and at times interchangable, and the third spot is in need of attention. How much attention Dark Knight needs is something people disagree on.
Why? Dark Knight has comparable dps to paladin on a striking dummy, but usually falls behind slightly when fights have down time. Paladin makes up for this lower dps compared to warrior with utility, it buys lots of healer uptime, but Dark Knight doesn't seem to bring much in the way as a counter balance here. Weird design issues: The tank which is hurt the least by down time has 2 knock back cancellations and a gap closer with a longer range all on shortish cooldowns, the tank hurt in the middle here has knockback prevention and no gap closer and powerful ranged attacks to minimize downtime, and dark knight is hurt the most by down time has no knockback prevention 1 gap closer with a shorter range and loses quite a bit of dps if you need to hold that gap closer and lacks a powerful ranged attack to minimize the uptime.
As for mitigation, Dark Knight does not have a native cooldown which lasts longer than 10 seconds. Most of its kit is set up well for tank busters, and if you find yourself low on HP has a powerful mitigation tool in the form of TBN. However, it offers little in physical turns and offers less overall mitigation on busters than paladin. Dark knight also suffers here on fluff damage because its short duration cooldowns do not necessarily cover damage outside of busters.
In terms of utility dark knight doesn't really offer much for groups, as compared to warrior and paladin. Warrior and Paladin are fairly comparable because though warrior is bringing more dps paladin is bringing a number of utilities that allow healers to contribute more to groups. The dps/utility allows healers to contribute more to groups.