Regarding the tank role, I would suggest you to level up all 3 to be able to fulfill all aspects of tanking.

PLD - "The Shield says: I Won't Break."
This is the best Tank Job to sustain damage from mobs.
You'll be hard to kill for them.
You won't suffer.

WAR - "The Sword says: The Best Defense Is A Good Offense."
This is the best Tank Job to damage on mobs.
You'll be killing them.
You'll make them suffer.

DRK - "The Spirit says: I shalt not recoil before mine enemies."
This is the best Tank Job to control multiple mobs.
You'll be targeted by them.
You need to suffer from them to make them suffer.


If you just want to tank for the fun of it (Casual: Roulettes/Dungeons), you can just pick up any of the tank jobs available.

If you want to tank for the challenge (Hardcore: Trial/Raids), overall, all tank jobs do around the same DPS during a Raid encounter (Debatable). Make sure your rotations are in check.