i beat the game solo with axton, got him to 50. played co op almost entirely with zero, did some parts by myself, got him to 50. maya mostly co op, but only level 44.
other 2 I haven't used much, but right now I'd say Zero>Maya>Axton. I like it that the skills are active, not a set and forget like the turret. Maya I really only played for support, but with both her and zero you're always involved. I tried respecing my axton several different ways and just don't have as much fun with him.
Gaige. I'm level 40 and I went Ordered Chaos, now things completely random happen around me. Deathtrap randomly blows people up, a bad guy nudges me as I'm shooting them and somebody blows up, I jump and fire a rocket launcher and I create a miniature nuclear blast. Also, my bullets can do 180s to hit people in the backs of their heads... or go flying off in a random direction to hit an enemy I can't even see. I ever tell you about that time I picked up a gun in Opportunity, test-fired it into the ground and the ricochet bounced into an engineer patrolling just around the corner? No clue he was back there, but he got shot. Watch it if you take close enough, you'll get some consistant hits at high levels of Anarchy but it can also get you into some serious doo-doo.
The word of the day is "Legs." Spread it around. GT: TimoInsano