Ace - shouldn't have replied to Akainu's provocation. I know plot wanted him to die(Oda wanted him to die since his introduction), but the way he did it was pure bullshit. Characterization-wise it made sense though since he is like Roger, who never turns his back against an enemy if he feels his pride, the thing he is fighting for is getting insulted, or the one he cares for is in danger(Luffy), but he shouldn't have done that. However Oda(plot) decided otherwise