First, skiing is has such long history and diversity, the saying someone is unequivocally the greatest, will alway be debatable. how do you compare a jibber to a racer? That being said, if i where to put down a name, it would probably be a big mountain skier, pushing the limits of what can be skied on the grandest scale. But the most important thing when looking at this is the the skier be held up against his peers.  You cant say, "if he had the modern day equipment and backround we do...." or ignore that either. You have to look at what that person did compared to the other top pros of the time, and with greatness in my opinion,  you also have to examine how that skier redefined what was possible on skis. In my opinion, for someone to be considered the greatest, they must redefine their sport in more than one way.  In skiing, (big mountain), i would look to see someone pushed what they skied in scale, technicality and creativity, how they skied it(style), and did all this continually.  Also, the greatest in a sport usually has that intangible skill by far and away the best, Look at michael jordan and tiger woods, I can think of no one better when it is on the line than these two.