Billy Bob Thornton -- The GREATEST BAND of All Time Is ...

... the Beatles. Duh.

BUT ... Billy Bob Thornton says if he HAD to name the greatest band of all time -- but couldn't pick the Fab 4 -- he knows exactly which group he would pick.

Hint: BBT's (second) favorite band was huuuge in the '70s ... but never scored a #1 hit on the Billboard Hot 100 list. They did have a #2 ... with the word "Man" in the title.

Second Hint: The group was founded (and named after) two brothers.