Gary Jules - Mad World. I never know what to think or feel when I listen to this song.
Aeris' theme (orchestrated version). If you played Final Fantasy VII, and you don't feel anything at all when you listen to this, then you have no soul.
Ralph Vaughan Williams - Fantasia (on a theme of Thomas Tallis). Okay, so even I'm not sure what the title precisely is, and I played the damn piece. Basically, it's a 16+ minute piece played entirely by strings, and each section (possibly minus the bass, it was a long time ago) is split into two different parts, AND each section has one leader, who gets a chance to play a super cool solo. Anyway, maybe it's because I played this (in high school, no less) but I think the effect it has on me can be best described as amplifying whatever emotions I'm feeling at the moment. Exultation, sadness, emptiness, awesomeness, and everything in between.
Pink Floyd - Uncomfortably Numb. The first time I really took a minute to listen to the lyrics, it haunted me. I had this image of a guy (musician, from what I inferred from the song) with a deadened look in his eye, strung out on whatever drugs he's had injected into him.
Led Zeppelin - Stairway to Heaven. I can't explain why, but there's something about the forlorn, lonely melancholy tone of the entire first half of this song that just gets to me, more than it probably should.