I like going back and thinking about that from time to time. Especially with bands that I've been listening to for years. Anyhow, here's some of my "first impressions" of bands that I love now.
Pinback - "Hurley"
The Kinks - "Picturebook" (well, actually the first Kinks song I heard was "Lola" but I didn't know it was them. When I heard "Picturebook," it was the first time I actually wanted to find out who they were.)
Broken Social Scene - "Stars and Sons"
Wilco - "I'm the Man That Loves You" (I feel like Jeff Tweedy, in this video, may have been the inspiration for Jack Black's character in School of Rock)
Beulah - "If We Can Land a Man on the Moon, Surely I Can Win Your Heart"
The Beach Boys - "Kokomo" (sung by the Muppets of course)
The Misfits - "Halloween"
Built to Spill - "Lie for a Lie" (no YouTube clip, but here's the audio)
Discover Simple, Private Sharing at Drop.io
The Mountain Goats - "Going to Georgia"