What is the epic song of a group?

King Crimson - Court of the Crimson King AND Starless.
Yes - Close to the Edge.
Rush - 2112.
Pink Floyd - Us and Them.
Van Der Graaf Generator - A Plague of the Lighthouse Keepers.
Can - Halleluhwah.
WAR - The World is a Ghetto.
Genesis - Suppers Ready.
Lynyrd Skynyrd - Free Bird.
Mike Oldfield - Tubular Bells.
Roxy Music - Avalon.
Henry Cow - Living in the Heart of the Beast.
Zappa - Way too many to list.....

Epic... in terms of production, sound quality, songs, etc... I'd vote for two Supertramp albums:  Crime of the Century of course, but also the less appreciated (post-Hodgson) Brother Where You Bound.  Several tracks are a musical journey, and reference quality sound production.  I've heard 3 different CD pressings of "Brother" over the years, and my first edition CD from 1985 is still the most dynamic.  Get it if you can! 

2nd,
WAR- The world is a Ghetto.

U2- Bad (live) - Wide awake in America.

The Rolling Stones - Midnight Rambler (live).