As a pianist, I have to disagree. While it is a great piece (and really easy to play - at least the first movement), there are soooo many pieces out there that are much better. Beethoven himself even once said "Surely I've written better pieces than that" in reference to the moonlight.
Anyway, in terms of serene quiet pieces, the first movement is up there, but if you want some really cool music, listen to his Appassionata Sonata or Tempest Sonata.
Then there's all those other composers...
Just a brief list of some of my favorites:
Brahms: 2nd Piano Concerto, Impromptus Op. 116, Op. 119 (Rhapsody especially)
Chopin: Nocturne in Cm, Op. 48 No.1; "Heroic" Polonaise in A-flat, Op. 53
Debussy: Almost all of the preludes, although Sunken Cathedral is a favorite, Claire de Lune, Arabesques, Images...
Liszt: Liebestraum No. 3 (although the entire set is really nice); Vall?e d'Obermann (big epic piece...I love playing it, even though it's really hard!)
Rachmaninoff: Prelude in Bm, Op. 23 No. 10 (I think), Melodie Op. 3, 2nd Piano Concerto (especially the 2nd movement)
Scriabin: Etude in C#m, Op. 2 No.1
and much much more - and this is only piano music
Anyway...sorry for that...it's been a while since I've had the chance to talk about classical music, and I miss it.