I'm still a little unsure on this one. Are all melee attacks from all heroes (by pressing V (default)) identical, in damage, hitbox, and range? Or are some melee attacks more damaging than others, or are there others that have more range than others?
With one exception, all quick melee attacks performed with the V key deal 30 damage and have the same hitbox. When playing as Brigitte, pressing V will swing her main weapon instead.
Some melee attacks, however, are their own weapons: Reinhardt's hammer deals 75 damage, Torbjorn's hammer deals 55 damage, Brigitte's flail deals 35 damage, Winston's attacks during Primal Rage deal 40 damage, and Genji's Dragonblade deals 120 damage.