As the bumper sticker says : "Gene Kelly did everything Fred Astaire did,backwards and in high heels".

I'd have to go with Gene Kelly.

both are good. each have their own unique style. Kelly being the more athletic and doing those wow moves. really liked his singing in the rain routine. Astaire was good at finding original ways to dance. such as with props and the appearance of dancing on the ceiling. wish I could dance like either of them.

Gene Kelly was the more athletic dancer. That's a given. He did some amazing stuff, but when it came to style, grace and class, Fred Astaire was the man. No one could make a partner look better than Astaire, he could and did make a broom look good. For my money Fred Astaire was the best. The rest could battle for second and third.

Gene Kelly was a gifted dancer, so it is no insult to say that he is not in the same class as Fred Astaire. As Irishgit and Frank have said....no one is.

Grace, elegance, style, versatility - Astaire.
Power, precision, flash, and probably a match in versatility - Kelly.

I'd have to go with Astaire, having been the guy who defined the form as far as dancing on the movie screen goes and for being the defining icon of the the great era of style that has disappeared from the landscape.