Bob Marley & the Wailers
Little Feat
Steely Dan
Honorable mention: The Doors
I don't rate their contributions to music as being greater than the likes of Elvis, The Beatles, Bob Dylan, etc., but for whatever reasons, I still get a little giddy whenever I hear their stuff.
Kind of cool that someone rated JJ Cale at the top of his list. He's another one that does something special for me. So soothing and relaxing. He's got this amazing command over an audience in a 'King of Cool' kind of way. JJ's stuff is about as mellow as it gets so he's definitely not for everyone, but he's quite spectacular in his unspectacular style. He just grabs a stool, sits down and starts strumming along and singing in raspy, muted tones. He almost mumbles at times, but somehow it all fits. No hype at all, just amazing talent. Unfortunately, most of his albums are poorly produced.