2. Pink Floyd
During the mid '60s, prog rock wasn't even a thought in the casual music fan's consciousness. If you were looking for something that was a bit more off-the-wall than the Rolling Stones or the Byrds, chances are you would find what you were looking for in the psychedelic genre. Pink Floyd seemed to fit perfectly in that scene at first, but things started to get a little bit different as the years wore on.
After the departure of founding member Syd Barrett due to drug-related mental health issues, the band persevered by stretching out their songs to unbelievable proportions. The idea of a 18-minute song seemed a bit daunting at the time, but these guys made every second count by crafting some of the most immaculate sounding records in rock like Animals and Wish You Were Here.
Aside from the long songs, Pink Floyd also expanded the horizons of mainstream rock, from the subtle details of the Dark Side of the Moon to the concept album to define all concept albums with The Wall. Prog may have still been a niche genre at the time, but Pink Floyd took it out of the cult doldrums and showed the music world that this elaborate style of music actually had staying power.