In terms of musical ideas, I think Beethoven was better; however, Chopin, in my opinion, had much more original piano figurations - more imagination in terms of the mechanism, per se. So, I suppose my answer depends on what you mean by 'best'.
In terms of sheer composition, I don't think Chopin can even compete with Beethoven (then again, only J.S. Bach can) - and Chopin is wonderful...gives you an idea of Beethoven's stature.