The Gunzerkers is objectively the most OP. Sirens are indispensable in teams because their abilities to give second winds to teammates long-range. Since the level buff, Commandos have gotten pretty OP late game with the ability to throw 2 slagging turrets, and never have to worry about weapon swapping. Assassins are good if you can chain sniper headshots for massive damage, and are good against bosses. However, the Mechromancer is fun, and stacking anarchy can do tons of damage, but at the expense of you not being able to hit the broadside of a barn, and Kreig's tankiness is offset by just how terrible he is in OP8 playthroughs.