I saw Hendrix in his prime. 1969.
I saw Zep in their prime 1969 and 1971.
I saw Jethro Tull in their prime. 1971.
I saw Yes in their prime (all the main key players). 1970.
I saw Elton John in his prime. Just after Yellow Brick Road.
I also saw The Doors just before their decline (so we'll call it their prime).
And I saw Blind Faith in their prime. Given they only had one album.