Of course Beethoven's Fifth is tops for me, and the Ninth close behind. I also like:
Also Sprach Zarathustra - Strauss
Fanfare for the Common Man - Aaron Copeland (what a nice thing that was for him to dedicate such a great piece of music to plebeians like me *sniff*)
Symphony #40 - Mozart
The Nutcracker - Tchaikovsky
I probably won't have much to add to this thread because I really don't know classical music that well, but my appreciation keeps growing. Ah, can't escape my dad's influence there, he's a conniseur.