"Three O'Clock Blues" (1951)
It wasn't B.B. King's first single — more like his ninth — but this cover version of Lowell Fulson's "Three O'Clock Blues" that King released in 1951 became his first and biggest hit, topping the R&B charts for five weeks. As crudely recorded as it is (King's singing and guitar are far louder than everything else), "Three O'Clock Blues" set the template for his career, with the rich, urbane tone of his voice answered after nearly every phrase by a cruelly prickly comment from his guitar, and an arrangement that's a bit fancier than the backwoods-shack country blues it suggests. There's even a horn section.