Monk

[Captain Maxx Power]

While a low job level onion knight won't do as much damage, I thought a 99/99 FEOK would do more damage than a 99/99 Black belt?

In terms of raw attack power, a FEOK will have an attack power of 312. By comparison a 99/99 Monk has an attack power of 396. Whether or not it equates to more damage in terms of the physical damage formula (which is so long-winded it defies belief) is another matter.