I would like to mention PLD, and WAR start at level 1 unless you buy a level skip potion.

DRK and GNB start at 30 and 60 respectively. DRK may have a leg up here in understanding the job over GNB since the level 60 dungeons are very wall to wall pull style, where the lower level dungeons from 30 tend to have more team oriented and job knowledge mechanics to them. Temple of Quarn, Brayflox Longstop, Dazmel Darkhold, etc.

I would like to also say PLD and WAR have really good opportunities to level and understand tanking in general with less emphasis on tense moments during dungeons.

WAR is the scrappy tank imo. At low levels it's great DPS that stays with it throughout it's leveling to 80. At 80 it is SOOOO good at staying alive, and from a healer point of view...I LOVE healing warriors. They life steal with Nascent Flash, large insta heal with equilibrium, and great high damage avoidance with Thrill of Battle. With mitigation that rivals other tanks WAR is very bombastic and never seems down for the count when against the ropes. WAR is the comeback tank to me.

PLD is THE turtle tank! At low levels it is very hard to get the upperhand on a paladin when tanking. They have the best mitigation in the game as far as a WHOLE kit goes. Sentinel, Sheltron, Divine Veil, Intervention, Passage of Arms, Hallowed Ground. It's VERY hard to kill a GOOD paladin, and even harder to drop a full group with paladin present. It's combo is separated with physical and then magic. Paladin is elegant to me.

DRK is just....very spring operated. It feels like it's wound up and then releases the pressure. The feeling of building pressure is amazing with DRK and when it unwinds? WOW! DRK mitigation is on the level of GNB or WAR, but with one glaring exception. The Blackest Night. This skill is the crux of DRK and is the single most best mitigation skill in the game for the tanks. Short cooldown, strong mitigation, and when preformed correctly rewards with a free oGCD damage buff/extender. DRK is VERY resource management heavy, but has great rewards if done well. DRK has to constantly be going back and forth with when to mitigate and when to damage. I would say this job needs the most practice and self control to play.

GNB is basically Devil May Cry in FFXIV and is crazy fast DPS. Continuation makes the class what it is and you don't get that until level 70. GNB is what happens when SE fat fingers the role button and hit's tank instead of DPS.