I like quite a few of the Chopin Waltzes. But right now, I think Op34 No1 gets the cake. But I love Op69 No1 aswell.
Favorite Beethoven Sonata? Tricky one! Not really sure, depends on the day. Pathetique is a favorite of mine, but so are many others. Op90 is nice. Op2 No1... so many good ones!
Rach preludes are easy - Op23 No4.
Not sure about the Bach P&F. I would have to go through them again, cuz I cant remember them all of hand.
Some pieces worth mentioning - Mendelssohn Songs without Words. There are some really nice ones, definately worth a listen. And there are loads too! I remember one in Op84 which was pretty sweet.
As for picking just one piece, sorry, dont think I could. Depends to much on my mood. Sometimes I would say a Beethoven Sonata, other times it would be a Chopin Waltz or Polonaise.
SJ