I noticed while leveling drk that at low levels, it's incredibly good. Specifically, it has an AoE attack that basically uses no resources (nothing else at that level uses MP), at a level where very few classes can AoE. In low level dungeons, I consistently top DpS charts as drk.
However, as drk gains more tools it becomes more complicated, and must worry about defense. While at the same time, the other tanks gain tools to overtake it. It's unable to AoE as much, and must use its resources more wisely. Eventually, Warrior overtakes it and is able to do what it does with less stress.

Paladin in the meantime really suffers performance-wise until it starts gaining its tools. It has difficulty with threat until 40 due to lack of enmity modifiers. And its runs are always slower, since it cannot do AoE damage. In the low level runs I previously mentioned, drk would be topping charts while pld can barely contribute to damage, while neither really need to be healed.


So, here's my list.
1-30, drk is best
1-40, pld is worst
54-60, war is best

I can't tell otherwise, though. At 40, tank damage drops and damage taken rises. So extra dps doesn't matter as much, while survivability matters more. Any time you greatly overgear a dungeon, Warrior will be better just because defense matters less and offense matters more, so they may be better at 50 dungeons. And after Warrior gains Deliverance, it becomes the most fluid tank. So perhaps, Warrior is best 50-60. Drk isn't clearly worse until deliverance. So a less accurate list would probably be:

1-49, drk is best
50-60, war is best

I want to fit Paladin in there somewhere, but it rarely gets a place to shine. It doesn't get easy until 40, it doesn't get any great tools until 50+, and it can never AoE well. Still, it handles rough situations better than the other tanks, so it's great for difficult dungeons. It wouldn't take me much convincing that:

1-40, drk is best
40-49, pld is best
50-60, war is best

This could go many ways. What do you think? What tanks are best at what levels? Are there levels where they're clearly even?