Death Penalty, without question. It's not that difficult to get Vincent up to 9999 damage per hit, which is consistent no matter what his HP/MP/Limit/Materia. Plus as Endless showed it can do ridiculous damage due to it's damage calculations. Conformer would come next since it does damage based on an enemy's level, which means that higher level enemies take more damage, which is sensible when you consider that, in most cases, higher levels means more HP.