I couldn't name just one as there are so many great songs so I have five, in no particular order:
The Tears Of A Clown - Smokey Robinson And The Miracles
If I Were A Carpenter - The Four Tops
I'm Still Waiting - Diana Ross
The Onion Song - Marvin Gaye and Tammi Terrell
Sir Duke - Stevie Wonder
If I were forced to choose just one of these five, I think I'd settle for The Tears Of A Clown. It took three years for it to be a UK hit. Motown's finest hour.