"Ooo Baby Baby" (Smokey Robinson and The Miracles)

Smokey Robinson wrote a slew of hits back in the day: "My Girl" and "Get Ready" for The Temptations, "Ain't That Peculiar" and "I'll Be Doggone" for Marvin Gaye, "My Guy" for Mary Wells, and a number of memorable songs for his own group, The Miracles, like "Tears of a Clown, "I Second That Emotion," and "You've Really Got a Hold on Me."
But nothing captures Smokey Robinson's genius in my opinion quite like The Miracles' single "Ooo Baby Baby" (1965).  It's a perfect synthesis of soul, lush orchestration, and Robinson's soft yet soaring vocals, which reside somewhere between the realm of opera and doo-wop.  It's exactly the kind of expressive vocal performance that made him a household name.
It would be incorrect, however, to just credit Robinson for the atmosphere of the song.  His writing partner and fellow member of The Miracles, Pete Moore, also deserves credit for handling the haunting vocal arrangements on the song, which are totally built upon a foundation of thick, jazzy harmonies.  The close intervals in The Miracles' harmonies are extremely difficult to pull off successfully, yet they make it sound like a day at the beach.
There has never been a better "I'm sorry" song than this.