Imo the player with highest dmd dealt should be the one who uses his personal weapon.
Actually I can't remind if I've never seen a tank or berserk becoming top dmg dealer.
It's either gunners (most cases) firesista with skyfire and sometimes refridgerator (although I'm not sure if instakill skill after freezing opponent counts as highest dmg).

At least it should be be split between highest dmg made with personal weapon and generic highest dmg.


edit: everything is related to "top experience per damage" at the match summary screen