I play the 3 tanks.
DRK is a lot of fun but can be pretty hard to learn as a first tanking job. It really comes to personnal preference. PLD is easy to learn and feels reliable and very smooth to play; WAR is slow-paced (not a lot of oGCDs to weave) but feels very aggressive, has a high skill-ceiling and is very rewarding when you master it. DRK is very fast-paced (a lot of oGCDs to weave, Blood Weapon which increases your attack speed) and feels very aggressive like WAR does but I believe that the skill-ceiling is not as high as for WAR (tho still higher than for PLD). DRK is also the only one that's able to jump on the enemies with Plunge (a gap closer ability), which feels absolutely great as a tank.
My fav ones are DRK and WAR. Originally I mained WAR before 3.0, but switched to DRK and love the shit out of the two of them.