Peter Green's Fleetwood Mac. Way before Stevie Nicks, Mac was the best blues band coming out of late 60's England. Better than Cream, certainly without Cream's ego trips. All-British, all blues, triple guitar lineup, Mac was a Guitar Hero's dream.
Peter Green, in Clapton's opinion, was on a par with BB King. Jeremy Spencer was a slide master who brought Elmore James to recognition. Onstage he also enlivened the act with 50's parodies. Danny Kirwan brought some jazzy explorations far beyond his tender 18 years.
This incarnation of Mac was only together about two years, but cranked out four albums, four hit singles, and in 1969 were outselling the Beatles and the Stones. That took some doing.