"Let's Stay Together" (Al Green)

There's just something about "Let's Stay Together" that never gets old.  I've heard it hundreds of times: on the radio, at wedding receptions, in film, and on T.V.  But it never wears out its welcome.  Ever.  Even when director Quentin Tarantino stretched the track's 3:30 running time to over 5 minutes for the scene in Pulp Fiction where Marsellus Wallace tells boxer Butch Coolidge to put his pride aside and take a dive, it still felt 5 minutes too short.
It's the combination of Al Green's sincere lyrics, delivered in his smooth-as-butter falsetto, and Willie Mitchell/Al Jackson, Jr.'s steady, jazz-inflected groove that seals the deal for me.  Nothing about it feels contrived.  And I think that's the key to the track's longevity: it's as pure and natural as water.
And apparently, the whole thing came together pretty naturally, too.
As Mitchell recounted in the 1987 documentary The Gospel According to Al Green, he was experimenting with some jazz chords on piano one afternoon in 1971, purposely looking for something more nuanced to smooth out Green's sound, when he came up with the melody for "Let's Stay Together."  After playing the composition through for Green, he told Mitchell to "give him five minutes" to write some lyrics.
"About 15 minutes later, Al comes back with some words, and we start messing with this song.  So about a week later, we put the track down, and that's when everything happened."