(Sorry about the long post, I just prefer the narrative form to lists)
From birth to thirteen years later, I listened to nothing but oldies--I only listened to music in the car and K-Earth 101 was the only station they played. Thus, my first defining band:
The Temptations (almost entirely due to their song "My Girl")
This lasted until I got to Junior High, when I discovered that my boom box had a radio option and I listened to nothing but hip hop for three years just to piss off the rest of my family (I liked the music, too, but looking back makes me think it was a rebellion in a small, pathetic way).
Tupac
Sophomore year of high school, my best friend from Jr. High came back to my hometown after three years in another state. Every time he'd come over to hang out at my house, he would switch the radio to a rock station, strap me to a chair, and drill me on the bands that were played ("Hear this? This is Pearl Jam. This is good music. Hear this? This is Nirvana, this is also good music." (OK, he didn't strap me to a chair, but he refused to listen to hip hop.)). He then got me as a gift my first punk cd
Blink 182 (shut up! I know you know all the words to "Dammit" and "Josie")
which shortly led to
Green Day
Of course, I think almost all high schoolers and college students go through this phase:
Jimi Hendrix
The Doors
Freshman year of college, internet radio (yay university internet connections!) introduced me to jazz in a big way.
John Coltrane
Digable Planets (rap that samples jazz, how cool is that!)
Then, last year in the fall, I was playing through my CDs (I had built a pretty good collection by then) and put on Pearl Jam's 'Yield'. I'd had it for some time, but for some reason, something clicked in my head and I couldn't listen to any other band for the entire semester. Since then, they've been my defining band against whom I measure all other bands.
Pearl Jam
Now, though, Ted Leo + Pharmacists is starting to give PJ some competition.
Nothing Nice to Say has been introducing me to some good punk music (esp. Against Me!). QC => indie music, etc. etc.