Flash enmity generation is based on weapon damage. It's actually pretty comparable to overpower, except overpower can benefit from the damage increases from unchained, berserk and other buffs like that, while there's nothing a paladin can do to make flash more effective.
The only benefit to flash is that you can spam it pretty much indefinitely by weaving in RB combos, while a war will run out of TP pretty quick if he does nothing but overpower. But that doesn't come close to making up the difference in burst group enmity generation that a warrior can get from all their buffs.
