Four Tops - Still Water (Love) 
It's hard to think of higher praise for song than this: this song was a chief inspiration for the sound of Marvin Gaye's What's Going On album. That sort of hypnotic soul that seems like it could go on forever, it's all here ... I mean ... what else is there to say?
Oh, I know, that this is one of the very few writing/producing collaborations between two of Motown's finest, Smokey Robinson and Frank Wilson, and while I'm sure this version is as good as it gets, I'd still love to hear some long lost demo with a Smokey vocal on it. Loop it up and let it play.