After looking through the database I could not believe Blackfoot wasn't anywhere.
Personally I'd rank his 1970 album Ultimate Prophecy in the top 5. Simply amazing stuff.
The Ultimate Prophecy was a great and deep concept album. It is clear that he has strong influence from country and psychedelic but the last half of the record was amazing. If any of you get a chance listen to it go for it. I think he deserves to be on the list as a Progressive artist.
Here's two songs from the 2nd half of Ultimate Prophecy, Cycles and Waiting to be Born.
Here's another song by JD Blackfoot (Not from Ultimate Prophecy). The Legend of Crazy Horse