who will win Green Lantern vs. Doctor Doom.?

Answer 1

Which GL? Kyle Rayner or Hal Jordan?

Doom would beat the old GL's easily, he's too smart for someone with such an obvious weakness (yellow)

He's too smart for rash, impulsive Kyle, too. Though the Lanterns have unlimited power, Doom managed to steal the Silver Surfer's power cosmic, which is just as great.

You need a few Justice Leaguers to take down Victor Von...

DOOM!!!

p.s. Hal Jordan as Parallax would defeat Doom instantly, he was just far too powerful. But those days are gone.

Answer 2

Which Green Lantern? Kyle "I'm younger, hipper and badass" Rainer? Hal "Am I Parallax or Green Lantern this week" Jordan? or John "I'm the stoic one from the Justice League" Stewart? (to name but a few)

Of those, Kyle Rainer's pretty much the only one that could beat Doom since the others have the weakness to yellow and the 24hr charge. Doom's smart enough to use those weaknesses to win the day.

EDIT: Call me old school, but I hate the way the new GL corps and everything is set up in the comics, so I haven't familiarized myself with it.