What is so good about the Appassionata?

That's a hard question to answer about any piece, but Appassionata is certainly widely recognized as a great piece.

Actual quote from Vladimir Ilyich Lenin:
"I know nothing more beautiful than the Appassionata, I could listen to it every day. Wonderful, transcendent music. It always makes me proud, perhaps naively and childishly so, to think that people can work such miracles!"

I don't if Lenin is an authority on the subject but yes it is generally considered to be the greatest piece of music ever written for piano.

It's a golden calf no question but it's a great golden calf

Ah right I see.
Maybe I'm just a little narrow-minded with respect to my tastes but I've listened to the Appassionata a couple times and it doesn't seem to click with me. I think (coincidentally Richard Wagner does too) the best Beethoven sonata is his 28th. It's just a fantastic piece of music all round and each movement is beautiful in its own unique way. Personally the second movement is my current favourite piece of music ever. It is to me on its own a justification for my ventures within the classical domain.
I'll hazard sounding pretentious and say that I think it could even be classified as one of the precursors to modernist music.

I think there could be more Haydn on this list. At least the "Lestat" sonata (made famous by Vampire movie:)
Could be there.(google tels me it is Sonata No.59 in E flat major Hob.XVI, 49)

How on earth are the Waldstein, sonata 32 in C Minor and the Hammerklavier below stuff like Schubert's 21st?

Are you saying that there should be 5 Beethovens in the top 10 or that the Schubert work is way too high? I do think that Schubert 21 is one of the most esteemed piano sonatas ever written.

I hope to see Paul Dukas's piano sonata on there. That work is a beast of a piano sonata, each movement is over 10 minutes long and it has four movements totaling around 47 minutes. It was finished in 1901 but sounds like a throwback to the mid-Romantics era.

I'm looking to learn more about classical music, more specifically solo piano. Could someone recommend some material?

You can't go wrong with Debussy.

Brian, is there any chance "Für Alina" or "Spiegel im Spiegel" by Pärt could make this list? Alina was his first composition in the Tintinnabuli style which is the foundation of pretty much everything he did since about the 1980's.

Paul, generally it takes a pretty long while for classical works to be established as claasic or important pieces, and that might be the issue with those two works. I could find a mention of "Für Alina" in any of the sources I use, and only one mention of "Spiegel im Spiegel". According to that source, the latter is a cello sonata. From what I gather, it seems that Part is more noted for orchestral and choral music than for solo piano music.

And Gray, what I'm attempting worth the lists is to list works that are widely recognized for their excellence, so without knowing what in particular you're looking for, I'll suggest checking out the works near the top of the list at the beginning of the thread. Beethoven and Chopin are probably the 2 greatest composers for solo piano.

Thanks Brian. For convienience's sake, are there any good compilatations you know of?