I've only got three characters running at the moment: Axton, Gaige, and Zer0. My most powerful build right now is Axton with his focus on the Survival skill tree and overall damage resistance -- along with a Rough Rider shield (no shield points, but massive health bonus), he's only died three times over the course of 23 levels. By far my most effective alt, and he's worked out better in UVHM than I ever thought possible. Axton is as close as I've come to an overpowered character.
Zer0? As much as I love to play him, he's a squishy gimmick alt, and he's far too focused on specialization to ever be considered overpowered.
Gaige could be a potential OP, but for the fact that her mech's AI is a fucking mess. He gets stuck in terrain, wanders off, loses targets, and sometimes doesn't respond at all. I am digging Gaige as a shock specialist, but she could be so much more effective with a little code tweaking.
I haven't touched the other characters yet so I can't comment on them.
The general consensus in the Borderlands community seems to be that Krieg is the most overpowered character in the game. He's next up on my list.