Flight of the Bumble Bee has to be one of the coolest classical compositions ever. Who could of ever imagined someone finding inspiration for a song by listening to a Bee buzz around, blows my mind.
Rite of Spring is probably one of the 1st classical pieces I heard that really got me listening seriously to classical music. Fire Bird Suite and the other one that I'm having trouble remembering right now by Stravinski are also cool (someome mentioned it earlier).
Beethovens music is timeless, I love all 9 symphonies and his piano sonotas and quartets.
Vivaldis 4 seasons is great.
I enjoy the works of Isaac Albinez, especially when there arrranged for guitar. Tango and Leyenda are awesome.
Must admit, never really been a fan of Mozart, I don't hate his stuff, just kinda "in the middle of road" about it, doesn't move me like ather stuff I've heard.