Mordecai, because his skill trees are by far the best, and his ability is the second most damaging (after Brick's) and the most useful (drops items, money and health, tiny recharge, multiple hits).
Got him to level 50 the other day. Obscene criticals reaching into 18,000+, fantastic accuracy and reload speeds, 13 second cooldown on the bird, and the most overpowered bonuses going.
Max Relentless + Deadly + Riotous Remedy + Killer, and you're invincible. Throw in Focus, Gun Crazy, Hair Trigger and Calibre and you do more damage than any other class.
One-shotting Badmutha Brutes and pretty much anything else with a revolver is amusing.