Wow, got to say, I'm surprised by how few gunzerkers I'm seeing. I've been playing as him lately and he is just a beast. He can't die and with dual-wielding can deal massive damage.
Seeing a lot of people that played Zero and seem to regret it. I'm around mid 20s with him now, focusing on sniping. I really enjoy him, but he relies on a team a lot more. Without some solid teammates to tank or distract, he wouldn't work nearly as well. He can deal some fantastic damage if you can reliably crit, love instant kills. Just wish there wasn't so much melee enemies that can jump across the map. I use his ability almost exclusively for escaping to find another good sniper spot. I admit, it isn't as fun as the other 3 abilities.
Siren is still my favorite though. Having a phaselock right in the middle of a cluster of enemies with every element exploding around it is hard to beat.