Are you looking for all nasty or beauty ?
If your obsessed with the baddest cichlid...male dovii or a tilapia buttikoferi.
Nastiest fish overall...Hoplia aimara...Cost $1,000.00 for 1.
As for oscar vs dempsey...Demseys are meaner and have a set of mild dentures , but the oscar just gets too big and massive.
I would say oscar would win.
In the end , who cares about which fish would win.
I would not let them fight if I owned both anyway. Cruel for one thing and I like my fish in perfect condition.