It isn't their best or most memorable song maybe 'epic' isn't the right catagory perhaps song that redefined a group or introduced something new. The wiki page on the song is an interesting read.
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.