Someone mentioned You Really Got Me for the Kinks. I would say that was their iconic song, but I think their "epic" would be Waterloo Sunset.
What is the epic song of a group?
I will start. I vote for Stairway to Heaven for Zep. Its grand with a great finish. I will throw in Salisbury by Uriah Heap. Ok lets see some posts of what you think is a groups masterpiece.