Can't believe no one replied to this. I was just rereading some CM stuff, and was going to make a CM vs. GL, but decided to search for one first (as a dutiful poster should).
Anyways, with some of the stuff Genis has pulled off, he could take any GL. What good is willpower when the man you're fighting can manipulate the electric synapses in your brain, effectively making you will yourself to die?
Genis Vell is nearly my favorite hero.His powers are limitless.He can enhance his strenght with no limit using the negabands, his invulnerability too, the same for his reflexes.He can teleport himself, fly at warp speeds, manipulate light to create force fields, energy blasts, etc.When he goes mad and become a soldier of Kree Imperial Army he wipe out an entire starship of Shi'Ars without effort, he was also able to withstand agains the Maestro's attacks and give him troubles.
Also there was a bad version of him that was nearly a god and use his powers on a higher level
He did do it with the help of Entropy and Ephiphany, it wasn't just him, he kind of went with the flow, and provided enough energy to start the big bang.
His evil alternate reality future clone self Fredd, was a total badass. So badass, he needed two D's in his name, because one wasn't evil enough.
Honestly, it doesn't matter what Green Lantern you use. "Cosmic awareness tells me you're vulnerable to...getting your ass kicked." And he can manipulate the energies in his brain, or suck all the ambient energy in the air out from around him, freezing him.
Though Hal does have a certain advantage, in that with his GL energies, he can make specific constructs, whereas Genis can make shields and stuff, he can't manipulate it into advanced hard light structures.