Without the Blade, Monster X is nearly impossible, especially when you have a computer like mine, that manages to freeze while playing Cave Story...
(With it, he's a total walk in the park)
But with full weapons, Ballos, by far. He had four phases, each harder than the last by a significant amount. Phases 1 and 3 are easy for me, but what really gets me are phases 2 and 4, considering how surprisingly active Ballos is during these points in the battle.
Or maybe it's just because I was controlling Quote with an analogue stick while fighting Ballos.