If you're in a party of tank+healer+range+caster, the best use of the limit break is on a giant trash pull because that will cut more time from the run than taking 2-4% off a boss. If for some reason you still have LB at the final boss, SMN>MCH>BRD>BLM. The MCH/BRD LB does less damage than the SMN/BLM LB, but it casts slightly faster and MCH/BRD sacrifices less normal action damage over the LB's cast and animation lock than the BLM does.