I'd say that knight levels faster than thief. Bard too.
Knight: Has 2 AoE's early enough into the levels so is a good mobber. Can also tank, which is highly needed for dungeons usually. And dungeons give fast exp.
Bard: Can also mob, though a bit slower. But instead has healing capability. This means that they can also heal in dungeons, which is still more needed than dps classes.
Mage would come third, since it can also AoE (best aoe even?), but has less def than a knight, no self healing, and is a bit less wanted. But more wanted than thief at least in dungeons.