Pink Triangle (even better if we get to count it with Falling for You, since they run into each other, but I suspect that would be cheating. Falling for You was the first song I think I ever loved, though, and is a close second.)
It's pretty funny that I got so attached to that one, really. I grew up in a pretty strict Southern Baptist household, and Weezer was my first exposure to real music, when I was 16. (Before that it was all Backstreet Boys and Celine Dion type stuff.) One of my favorite memories from freshman year of college is when one of my friends played it for me, late at night, on my Baptist college's radio show. (I went to the kind of school where they sent kids to Gay-hab, and fined you for cursing, and had weekly chapel, so to us, at 17, it was pretty daring.)
At first, I thought that I liked it because it felt a little scandalous. (Lesbians? and smoking?!) I didn't really think I identified with it (or many of their songs, for that matter), being straight, a virgin, and a teetotaler. As I started to come into my own, however, it really started to speak to me. He was so vulnerable, so honest, so upfront about his idiosyncrasies. He was unapologetic, and yet self-effacing. It (and Falling for You) evoked more intimate feelings from me than any person ever had before.
and his guitar... He said so many things to me, and took me places I'd never been before. When he makes his guitar shudder during the solo, I still get chills sometimes. There's a sweet anguish there, and raw yearning, and that bittersweet resignation at the end. I felt the same things coursing around inside of me, but I had no vehicle to express them, nor could I have articulated them so well.
So, that's why Pink Triangle. God bless Rivers and his guitar (and the rest of the guys, too.) It's been a love affair ever since.
I know that's probably more than you wanted, but I just saw them at Voodoo (and I met Brian Bell!! and promptly turned into a mute idiot...) and I'm still on a Weezer high. To save you some reading, I won't even get started on FFY. ; )