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.
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I wouldn't clump entire classes into one build at all. Just for example, I would much rather have intellect/discipline on a bladedancer and discipline/strength on a nightstalker.