I'm gonna start a little riot with this one, but:

1) Summoner
2) Black Mage
3) Monk, Bard and Dragoon tied

Simply for the fact that anyone can DPS, but not everyone can master said class. Monk and dragoon rotations become muscle memory with enough practice. Bards have a certain buffing order.

Black mage needs to plan ahead to optimise DPS. They may waste a thundercloud or firestarter proc while they're saving it to use while moving. Ideally you'd want that last MP tick to happen right before you switch from Umbral ice back to Astral fire.

As for summoner. Anyone can slap some DoTs on the target and deal decent damage. But it's possibly the hardest class to deal as much damage as possible. It's not simply DoTing up when they're about to run out. They don't have a specific rotation either, unlike the other classes. You add your dots and everything afterwards is pretty much on the fly. Muscle memory simply doesn't apply for Summoner.