For sheer damage output, I'd put Iron Heart, Tiger Claw and (situationally) Diamond Mind at the top. For the first two, those "attack X times" maneuvers can really put in work, and straight damage bonuses are nothing to sneeze at, either. Diamond Mind shines in the damage department due to the [insert random mineral][insert random synonym of "knife"] maneuvers.
Iron Heart and Diamond Mind are also among the best for staying in a fight (Iron Heart Surge, save replacers), and have some nice stances (Hearing the Air). But for staying power, Devoted Spirit is probably on par overall, and depending on alignment, it can come up with very decent damage, as well (Divine Surge is brutal for the level).
But most powerful discipline, overall, all things considered? White Raven. Because it turns not only you, but your whole team into killing machines, and that is priceless. It also potentially breaks the action economy, which is priceless-er. Best capstone, as well, IMO.