PLD by far is the best dungeon tank in terms of you getting a random healer. Large pulls you can Hallowed Ground the first big pull and last big pull. And if your healer is bad you can Requescat and clemency for 70- 90k health instantly. DRK is second with TBN being so powerful but cant fully save you if you have a undergeared healer, with WAR falling to 3rd and GNB last for dungeons.

When comparing tanks for dungeons you have to look at both healing side and tank side.

However i have noticed if you are in a group without a WHM dungeons become slightly more challenging without the stun from Holy Spam.