I'll go with the first supergroup IMO- CSN&Y. They have been at it certainly longer than any supergroup I can think of even though they have split off and done their own thing over the years. A lot of people listening to their music now are probably not aware of their status as a supergroup. The Byrds, The Hollies and Buffalo Springfield were still at or near their peak when CSN&Y formed.

Many of the so-called supergroups that have formed since then are from members of groups past their prime or were formed after a key member of an existing group left and legally the remaining members cannot continue to use the original band name.

I'll also put in a quick vote for Blind Faith however they were not around very long so they barely qualify as a group.