Now do you want mainstream? (dc/marvel) or do you want to include other comic universes? (Chaos!, Anarchy, Wildstorm..etc...etc)
Well, Ill go based on mainstream:
Most Powerful All Around:
Superman- Strength, heat vision, cold breath, superspeed, plus he never has to eat or poo, pretty strong there.
Green Lantern- quickly catching up to Supes, if not already suprassing him, in power. He no longer needs the ring, he no longer needs to recharge. He can create entire worlds (living breathing actual worlds) with millions of completely individual beings populating it, and extend little effort.
Captain Marvel- About tied with supes, really. I might even put him at a slight advantage based on the nature of his powers (magic).
Martian Manhunter- Yeah, this guy is badass. Cept for that fire weakness thing...
in Marvel:
Quasar- Like green lantern before he(gl) got his massive powerup
Hulk- He may not have flight, or x-ray vision, or super speed, but NOBODY is stronger than the Hulk. His strength has the potential to be completely infinite. He has grabbed and broken energy, lifted an entire mountain, and kept an antimatter bomb from going off all with pure strength. The madder Hulk gets, the stronger Hulk gets.
Thor- Id put him on par with Superman, specially now that he has the Odinforce.