I'm talking like a song that you heard that you instantly fell in love with and maybe that got you really into a band.
I'd have to say the song "Nutshell" by Alice in Chains was the biggest one for me.
The minute I heard this song I was in love with it. The lyrics just hit me and Layne puts a lot of emotion into it. Plus after I heard this song, I fell in love with Alice In Chains. Heard the song over a million times and I still love it.