Cyclops -- Scott may have been young, but he had a grasp on his power -- already having the ability to shorten and widen out the beam depending on the task at hand or number of enemies. He's the definition of calm, cool and collected, and though he would never want to hurt his friends, he wouldn't hold back in the pursuit of bettering himself and his teammates. Should he remove the visor completely, which he rarely does, the blast would be powerful enough to disintegrate body parts and walls.
Marvel Girl -- Jean's telekinetic powers were still untapped at the time. She could lift smaller objects (i.e. books, knives) and push/pull others (i.e. doors, Xavier), but that's about it. If possessed, someone could force/use her to fling hundreds of metal shards or knives at her teammates -- but this isn't the case.
Beast -- Agile and smart, but back then (still?) he abhorred violence -- even in training exercises. Hank did fight when it was for a cause (RE: Xavier's dream), but for the most part he jumps around a lot. That might make him somewhat evasive, but not so much when his teammates have optic blasts, TK, ice powers, and wings.
Angel -- Warren's wings make him as agile as Beast, if not more so, but what can he really do when it comes to fighting? Once in a while he was able to create small wind gusts with his wings, knocking opponents off their feet -- but that's not going to win a fight. Tack on his metal wings and his chances rise, but his natural feather wings can too easily be harmed by fire or projectiles.
Iceman -- Back then Bobby didn't do much more than turn into a snowman, throw ice/snowballs, and cover people in frost. He had a few tricks up his frozen sleeves, but not enough to win.
Winner: Cyclops