What is the epic song of a group?

Roberjerman it’s “The Byrds”, not “The Byrd’s”. 

Elton John, “Burn Down the Mission”
Billy Joel, “We Didn’t Start the Fire”
and +1 for the Beatles, “A Day In The Life”

In no particular order....
Eagles - Hotel California
Molly Hatchet- Gator Country
Skynyrd- Free Bird
Patsy Cline - Crazy
Paul Simon - Late in the Evening
Frank Sinatra - That's Life
David Bromberg - Cocaine Blues
Linda Ronstadt - Long Long Time
Stuff - Do You Want Some of This?
Doc Watson - Sadie
Eris Clapton - Layla
New Grass Revival - Lee Highway Blues
Bobby Bland - Stormy Monday Blues
Temptations - My Girl
Wilson Pickett - Mustang Sally
Elton John- Take Me to the Pilot

I now realize many of us, certainly I, didn't take the "epic" qualifier seriously enough in our nominations.

Talking Heads-Burning Down the HouseEagles-DesperadoRolling Stones-Ruby TuesdayBob Dylan-Lay Lady Lay
Jethro Tull-Locomotive BreathPink Floyd-Us and ThemBeatles-Let It BeElvis-Jailhouse RockMoody Blues-Nights in White SatinSimon and Garfunkel-Scarborough FairPeter, Paul and Mary-Blowin' in the WindMamas and Papas-California Dreamin'Cars-DriveHeart-Crazy on YouPrince-When Doves CryDire Straights-Brothers in ArmsJudy Collins-Amazing GraceAretha-ThinkLinda Ronstadt-Rock Me on the WaterSteely Dan-Rickie Don't Lose That Number