1. Alice Cooper in Wayne's World
Everyone remembers this scene for Alice's mild-mannered discourse, but the part that I could never get over is that Wayne and Garth head backstage while Alice is still playing "Feed My Frankenstein."
2. GWAR in Empire Records
Ever try to explain GWAR to someone? It never works, but I still try, and I usually start by asking if they've seen this movie. Easily the best scene. (Also, didn't realize until now that Daniel Johnston's "Rock 'n' Roll/EGA" is juxtaposed with "Saddam A-Go-Go" here. Brilliant.)
3. James Hetfield in South Park: Bigger, Longer and Uncut
Metallica's frontman tells Kenny that he's going to Hell, the consolation being that he'll still get better music than the angels.
4-5. Ronnie James Dio and Dave Grohl in Tenacious D in The Pick of Destiny
One terrible movie bookended by a Ronnie James Dio poster coming to life and Dave Grohl playing one of the all-time best Beelzebubs. YouTube clips were invented for movies like this.