Yeah your probably more correct calling GRAND FUNK RAILROAD at onetime America's most popular rock band rather than greatest rock band that's my sloppy writing Slick Willie.
Personally speaking I have heard all of 'the funks' 1970s albums and as good as they are they tend to have a slight soul/rock feel to them instead of a all out rock feel to them which is ok if you like the slightly more soulful element in rock music. My favourite Grand Funk album is the 1976 album GOOD SINGIN' GOOD PLAYIN' which was produced by Frank Zappa.