Im not really into classical music or anything, but recently i heard Beethovens Moonlight Sonata, and i think it is the most beautiful piece of music ever written.
If you seriously like classical stuff then I suggest that you try listening to few classical raags and you will get hooked. We listen to them all the time as they have been copied umpteen times in various movie songs - sometimes really good versions and often a total mess of the original...
I am personally a big fan of Ravi Shankar, Ustad Salamat Ali Khan and Ustad Nafees Khan. The Sitar Work is just out of this world....