I think probably Kirby, since he could just swallow Link up and imitate all of his moves.
Maybe I should simulate this in Brawl. I would do a CPU vs CPU match 100 times, with Link vs Kirby.
That seems to be player vs CPU, so I don't think it is really a fair comparison. Player vs Player would not be fair either, since some players are less skilled than others. The only fair way to do it would be CPU vs CPU, with the same settings for both players, and a generic stage (like Final Destination) multiple times.