Maybe it's just because I started GLA/PLD, but I still think PLD is the best "tank" between it and GNB/DRK. Those two feel like they're trying to be DPS classes with all of their oGCDs, as well as losing the shield's HUGE extra defense. PLD just feels like an solid rock, because it doesn't need gimmicky abilities to shore up parry/block or mitigate Mag/Phys dmg.
DRK and GNB are really squishy, especially if their healer isnt on point. With PLD, Reprisal and Sheltron are on such low CDs that even with slow healers you'll never have to worry about pulling big or being too slow out of an AOE.

This is strictly my noob opinion, but I also feel like the upgraded Halone's Wrath and the oGCD aoe move giving you two DOTs lets you focus a little more on timing your Sheltron or the party shield move (Which I still haven't used seriously outside of Alexander, smh) rather than making sure your rotation is on point.