Oh wow... Um... I don't think I've ever gotten 100% on a song in guitar hero. Probably because I play rock band more though. I think mg first 100% was "Welcome Home" by Coheed and Cambria on medium, but I think my first 100% was (maybe) either "21 Guns" on Green Day rock band or "Twist and Shout" on Beatles Rock band
Hmmmm... never lol. I never got good at Guitar Hero but the closest I got to was Hard mode in GH3 and that was Rock You You Like a Hurricane by Scorpions.
I started playing the franchise somewhat late starting from World Tour (GH4), but I believe the first song I beat on Expert was What I've Done by Linkin Park.