I think it was Troublemaker... I know that I had heard of Weezer long before I actually started getting into their music, but Troublemaker was the first that I looked up. That's when I realized that Weezer is freakin' awesome! It's still probably my favorite. Lol, my iPod says that I've played that song about 1,518 times!
I remember hearing Undone as a very small child (maybe 4) and talking extensively about it with my older brother, I'm pretty sure we had Blue on cassette at one time and listened to it while driving to Disney World in 99'. Although it wasn't the first time I heard it I don't remember much before the age of 4 or 5.