Mike Silver calls it that "certain something" and wrote a song about it... the subject was James Taylor in that instance. For me I think it's a certain soulful, heartfelt feeling in the vocals - as exemplified by Bonnie Raitt, Linda Ronstadt (on their better selections) and many others. Paul Simon as well, on his more acoustic offerings. And that guy who sang "Ain't no sunshine when she's gone"...

That's mostly from the vocals aspect. As far as rhythm/groove goes, Stevie Wonder's "Superstition" has to be up there among the top contenders for me!

Cheers - Tony