I picked "The Tears of a Clown" because the song literally changed the course of Smokey Robinson's life.
"The Tears of a Clown" was originally released on the album "Make It Happen" in 1967, music written by Stevie Wonder and Hank Cosby, lyrics by Smokey Robinson. By 1969, Robinson had intended to leave the Miracles, citing a desire to spend more time with the family in Detroit and concentrate on his job as VP at Motown Records.
The Miracles were very big in the UK, and with the group on the verge of breaking up, Motown was forced to search the back catalogue for a song to release. They remixed and released "The Tears of a Clown", and the song went to No. 1 on the charts. Looking to duplicate the success, Motown released it over here...
This new found popularity prompted Motown to release the song as a single in the United States, where it became a number-one hit on both the pop and R&B charts within two months of its release. Despite the fact that The Miracles had been one of Motown's premier acts in the early and mid-1960s and its first successful group act, "The Tears of a Clown" was their first and only #1 hit while Smokey Robinson was lead singer.
This kept Smokey in the band for another two years. On July 16th, 1972, Robinson introduced new lead singer Billy Griffin at a farewell concert in Washington DC.
Song info thanks to Wiki, and you can read more on the long history of the Miracles here.