On ABC Classic FM at the moment is a countdown of the top 100 Classical Piano pieces, as voted for in a large listener/reader survey.
http://www.abc.net.au/classic/classic100/countdown.htm
There's some beautiful music in that list! I'm expecting to see some more Chopin and at least two, possibly four Beethoven Sonatas to come in the top 21 (at the time of posting, the list went down to No. 22). I would have voted for Chopin's Ballade in G minor, which I'm really hoping to at least see in the top 21, but I suspect something like the Moonlight, Pathetique, or Appassionata Sonata might be No. 1; or Claire de Lune; or maybe Mozart's K331.
Also, three years ago there was a similar countdown of any classical music (classical - genre - not period).
http://www.abc.net.au/classic/classic100/previous.htm
The list had a few surprises, and some obvious classics right up there. I was surprised to see what was No. 1, in fact.
Thought this would make an interesting discussion point!