Hunter: Intellect strength. Throwing knives are invaluable and their supers are the best on pvp. Before everything was nerfed. Titan: Intellect discipline. Grenades are good and supers are useful. Defender especially benefits the most it's downright annoying to the enemy. Warlock: Discipline strength. Depends on your build warlocks usually benefit a lot from melee such as flame shield or energy drain. They synergies and refill the super with their own benefit.