It goes by so quickly, you may as well listen to it twice!
For me it was Moving Pictures, especially Red Barchetta. First heard it when I was 10 or 11. I've gradually moved on from most of the music I got into back then, but my appreciation for Rush has only gotten stronger.
I've told the story here before, but my dad borrowed the album from someone, and I made a tape for myself by sticking a boom box in front of one of the speakers. I didn't know what "stereo" meant. I listened to it all the time. When I finally got a real copy of the album in my late teens I was blown away by all the extra parts I'd been missing.