Level 37 Zero.
I agree with a lot of what people are saying about his skill before you can max out a tree. It's hard to use effectively, doesn't make much sense with sniper rifles, and tends to leave you sitting in the middle of a huge pack. I got a lot of mileage out of his melee attack early in the game, and Execute kept it competitive (note that it works even when you're out of range, and can be used repeatedly to move really, really fast towards an enemy), but pretty soon after level 20 it starts falling off in favor of using the Deception bonus on point-blank shotguns, and even there I relied on being able to second wind more often than not. But shotgun Zero with melee finishers was by far the strongest build I had until level 31, and was pretty consistently able to drop the strongest enemy in an encounter instantly. Bosses were also generally very easy with a low delay shield. Ammo was a problem like some people have said, but that's why you carry a sniper rifle or rocket launcher, a shotgun, an assault rifle, and an SMG.
Many Must Fall leaves something to be desired as a combat skill; it's hard to chain since the damage bonus out of Deception is based on time spent in Deception rather than time left in Deception (the refresh is only 3 seconds or so, so if you get a full-strength attack out of the initial 5s cooldown, your next attack isn't going to be nearly as strong). However, Death Blossom is phenomenal, and can weaken a group of enemies or kill a single Super Badass by itself. And then you still get to shotgun blast out of Deception. Elemental SMGs also become really appealing due to Two Fang and Rising Shot. I feel like he's very effective at my current level, and I'm not having much trouble against same-level or slightly stronger enemies.
That said, there have been some pretty boring stretches. He's nowhere near as fun as Lilith was. I'm looking forward to rolling Maya in a few weeks when my friends get the game.