Did Frederic Chopin best understand the intrinsic nature and potential of the piano as a expressive instrument?
Well, Beethoven wrote for the piano orchestrally. Schubert wrote for the piano vocally. But possibly the first person to write for the piano truly pianistically was Frederick Chopin. There have been many others, of course, but Chopin was the first really great composer to understand, in your words, the "nature and potential of the piano as an expressive instrument".