For DPS in Borderlands 2, the most powerful classes to play are Zer0 the Assassin, or Salvador the Gunzerker. Zer0 deals lethal damage in melee, while Salvador can wield 2 guns.
Zer0 is especially good in co-op, since he is a good finisher against single targets. This makes him very useful against bosses who have lots of health. Invest skill points in the Bloodshed tree.
For Salvador, investing in the Rampage tree is best for DPS.
