What's the best tank at each level?

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?

Hmm, to be honest i had a hard time as DRK between lvl 30 - 35 <--> after grit & darkside - before blood price.
I had good time as gladiator as well, fun class for <50 lvl duns.
I had good time also as marauder (with invigorate as cross class), and later warrior with defiance.

I quess thats it. DRK 30-35 was rather though for this early leveling.

Well, this is gonna be a little off-topic partially, as I am gonna just answer the topic mostly from a personal enjoyment point of view.

I tend to play DRK a lot when doing sub-level 60 content (especially the level 50 dungeons -roulette). It's just satisfying to do sprint pulls on it whilst hauling ass DPS-wise, and the kit works nicely even at a low level (provided one has Blood Price at least). I still enjoy DRK at 60 a lot and play it on occassion, provided I have the opportunity to do so (outside of just roulettes and dungeons).

At 60, however, I enjoy WAR a bit more and raid on it - and as such I enjoy it the most and also think it is the "best" tank at said level. As to why, it has been argued to death (utility, self-DPS, not having a weak fight to it, etc). WAR is competent early on, but the lack of a direct TP refresh before 58 and the lack of stack-ability-GCDs out of Defiance (for TP breaks) before 54 is kind of dull when you've gotten used to the level 60 WAR. As such, I don't play it a lot sub-60.

PLD is viable and I like to whip it out for bosses (trials etc.) here and there, but it's honestly not very interesting to me; not hating, just playstyle-related stuff. For dungeons, it just doesn't have that AOE for speedrunning - and also - its sprint pulls feel less fun than on DRK (Flashing vs. DRK magical attacks). For bosses, I enjoy its rotation much less than the other tanks'. That is not to hate on PLD's viability, but more about my tastes I guess.

So to summarize, I enjoy playing DRK and feel powerful on it below the level cap. At 60, playing WAR feels the most satisfying overall. I think PLD is okay and viable, but it isn't my go-to tank for any purpose. None of the tanks feel particularly weak to me at any level. Most of this has to do with my tastes, of course.

1-30 Marauder
30-60 Warrior

Depends on what you mean by best. If you're looking to tank with the minimum amount of "stress", as you say, then I would go with:
1-29: MRD
30-39: WAR
40-51: PLD
52-60: WAR

All of WAR's expansion abilities are quality-of-life improvements, so it ends up being one of two jobs which becomes paradoxically easier to play once you get to HW.