Really it was Undone, but I didn't pay much attention to that when I was younger. The first time I actively paid attention to Weezer and loved what I was hearing was Buddy Holly on the Windows 95 disc.
El Scorcho would be the first song I heard that I knew was Weezer. The "I'm a lot like you" bit really stuck in my thirteen year old brain. I bought Pinkerton and then was very excited to discover the Blue album. I did not know Buddy Holly and Say It Ain't So were their songs but immediately recognized them. After Pinkerton, I thought I would never hear from my favorite band again.... then, in high school, the Green album came out! I'm hooked for life. It's a good thing.
Why Bother? was the first one I can remember hearing, it was on my uncles crappy mp3 player and I loved it, but at the time I was so young I had no idea what the lyrics meant. To this day I still love that song
When I was about 8 years old my Dad would play "The Hip Hip Song" (that's what I knew it as) on guitar, and I heard Beverly Hills a lot on the radio, but Pork and Beans was the first song I heard knowing that it was Weezer. I thought it was the greatest song I had ever heard, and I instantly fell in love with the band.
undone (the sweater song) on the radio when I was in 8th grade. All I listened to before was Nirvana. After I saw the video on MTV, I bought a guitar magazine that had the tab. My band played it at a school talent show, but I think the judges were mad b/c they wanted us to do Buddy Holly instead. My guitar teacher thought undone was a cover of the Guess Who song...even when I played the cassette for him. This made me think that the sweater song was a cover for a good year, until I heard the Guess Who's undone, and had a good laugh.