First, those with no chance:
Archangel
Beast
Jubilee
Storm
Gambit in his most powerful incarnation would have a very good chance to win, but he has since lost those powers.
Rogue, same deal.
Colossus can’t win, because there are too many people who can take him out who he can’t beat, like Nightcrawler, Shadowcat, or Iceman.
Wolverine would be very difficult to defeat, because his healing factor is extremely powerful. But he essentially has the same problem as Colossus.
So who is left?
Cyclops
Shadowcat
Nightcrawler
Iceman
Phoenix
Cyclops can start off with a full power, wide angle, mountain levelling blast that takes out almost everyone else.
Shadowcat will phase, and then start phasing people into the ground like she did against Thor.
Nightcrawler will start teleporting around teleporting heads off.
Iceman goes to full ice form and tries to freeze all the water in his opponents’ bodies.
Phoenix puts up a shield and attacks people with both telekinetic and telepathic attacks.
Of these, the first to go down is Nightcrawler, as he cannot fully escape from both Cyclops and Iceman’s big attacks no matter where he teleports.
Shadowcat has a problem. I remember she once got knocked out by poison gas while phased. After all, she still has to breath. In this case the air will be either freezing from Iceman, or superheated from Cyclops, so she has to hold her breath. Iceman can dodge her attacks while she runs around holding her breath, but she may take down Cyclops before she goes down.
Final three:
Cyclops
Iceman
Phoenix
Cyclops can destroy either or both opponents with a huge blast if they aren’t prepared.
Iceman can possibly reform from water molecules and freeze both opponents.
Jean might be able to put up a shield to deflect the blast, throw Cyclops against the wall, and disperse Iceman’s consciousness so he can’t reform.
Any of the three can win this, so that I have a hard time even saying who would win more often. Probably Jean. But then again…