What is the epic song of a group?

Brand Funk Railroad. "Lonliness" over "Closer to Home" for me

Smoke on the Water - Deep Purple;
Sultans of Swing (especially the Alchemy live version) - Dire Straits;
Whole lotta Rosie - AC/DC; 
Psycho Killer - Talking Heads;
Contentores - Xutos&Pontapés;
Chiclete - Táxi. 

Janis Joplin "Me & Bobby McGee"
Aretha Franklin "Respect"
Bill Withers "Use Me"
Otis Redding.... the above....
Isaac Hayes "Theme from Shaft"
Peter Frampton "Do You Feel Like We Do"
Peter Gabriel "Sledgehammer"
T Rex "Bang a Gong"
Mountain "Mississippi Queen"
Ram Jam  "Black Betty"

I decided to forgo the major artists that have decades of output. (Where do you start?)

Private Sorrow/Balloon Burning - Pretty Things