'Oraison' by Olivier Messiaen. The best recording is from the original from the 1930's, played on Ondes Martenot. I am a total sucker for Piano music too (my brother is a virtuoso pianist and I was brought up with it) but I think pretty much any Messiaen takes it for me.
Schoenberg's very early atonal work ('Drei Klavierstucke') really is something too. Especially the second piece, I much prefer the expressionism to his more traditional work.