Alright, I have a few...
Ravel's Piano Concerto in G Major
Simply put, my jaw dropped at this one. I was so amazed by it. It was so different, so strange. It confused me. It intrigued me. I didn't understand it...but I wanted to. It was a piece of music that truly struck me as both original and powerful.
Someday The Dream Will End By Masashi Hamauzu and Nobuo Uematsu
I think I may be the only one here who will cite a track from a videogame as a life-changing one.
But simply put, the first time I played through FFX and it got to the moment when this track played, it simply blew me away. For those who have played, they'll know that this is pretty much the climax of the game. And the music definetly play a role. It's just...greatness. The way I felt the first time I heard it is the entire reason why I want to be a videogame composer. To do for other people what this piece did for me.
You may all laugh now.
Debussy's Arabesque No. 1
This piece is what made me fall in love with Impressionism and draw upon it as an influence.
Boulez's Le soleil des eaux
Well, I saw no mention of this thread having to be about pieces of music that impacted your life in a *good* way so I'll mention this one.
Simply put, this scared the s*** out of me the first time I heard it and ever since, I have been scared of anything with the label "12 tone" or "atonal" on it.