As someone with all tanks lvl 80 and played them in end game here is the list:

1) PLD simply due to it has a straight forward rotation and gameplay loop and is generally the best tank in the game right now.
2) GNB simply due to the fast paced nature and fairly close to a DPS rotation that is somewhat engaging gameplay. Lacks some survivability.

The worst two are:
3) DRK there is a bit more work required to manage mana so you can have your personal cooldown ready with a good chunk of OGCDs but the reward for doing it all isn't worth the investment with the lowest damage of all the tanks for no reason. Plus, Living dead is the worst tank invuln in the game.
4) WAR- Dear god WAR sucks. With raid buffs WAR drops behind DRK but honestly you can understand why when you play it. It feels like you are playing a class with wheelchairs suck to its bum with training wheels as legs. Good lord is it bland. You are solely reliant on IR to have fun with playing WAR. Once IR is on cooldown it feels like you are pulling teeth at how boring it plays. There is one button you hit way too much right now and it kills the job.