In chronological order:
Down On The Corner - Creedence Clearwater Revival
First "favourite" song, I was like 3 or 4 years old.
Blackened - Metallica
First metal song I ever heard, was five years old and I loved it.
Killing In The Name - Rage Against The Machine
First song I heard with swearing in it, I was 8 or so but I thought it was awesome.
Fight For Your Right - Beastie Boys
My first real exposure to hip-hop which would later develop further.
Got The Life - Korn
Got me into the obligatory teenage nu-metal phase that I'd rather not talk about.
F*ck The Police - NWA
Began my rap phase which lasted from 15-17.
...But Home Is Nowhere - AFI
Took me out of my rap phase and brought me into a lot of emo type stuff, Fall Out Boy (before they were awful), Something Corporate, HIM etc.
Turn Soonest To The Sea - Protest The Hero (21-current)
was pretty life changing when I heard it and sent me down the sort of music I'm listening to now. It wasn't like any of the other stuff I was listening to, it wasn't melodic, it was all over the place and the lyrics had a lot of meaning (albeit being somewhat abstract).