220px-Let's_Get_It_On#11 – Marvin Gaye – “Let’s Get It On” – (1973)

Those first few notes of this song tell you exactly what it is – and what it is is smoooth. This Hot 100 #1 is one of the sexiest songs ever and is soul defined. A lot of songs can lay claim to being Marvin Gaye’s signature song but I think this one might have the strongest argument.