I love Beethoven, Rachmaninov and Brahms in their entirety but, if I had to name my favourite classical piece, I would go for Smetana's "Ma Vlast" and particularly for the "Vltava" section.
On a more contemporary note, I think that it's probably well known that I adore much of Jon Lord's classical work, having bought them all from "Sarabande" onwards, in '76.