With the first, I was 18 and he was 19... "Wish You Were Here" by Pink Floyd... he introduced me to Pink Floyd, Led Zepplin, etc.
With the second, I was 19 and he was 23... "More than a Feeling" by Boston... he introduced me to AC/DC, KISS, 80's rock bands, etc.
With the third, I was 22 and he was 23... "Beautiful" by Pharrell... he introduced me to hip hop.
Hrm... I learned a lot about music via the ex's.

Used to listen to a lot of punk/OPIV/etc. and easy listening (Karen Carpenter, Neil Diamond, Elton John, etc.) on my own... branched out upon dating.
No song right now, he is 22 and I am 23.