I love Sly Stones music, the man is a genius. He fused the sounds of black and white music together before Michael Jackson and Prince. Though they are unique sounding songs, because Michael Jackson and Prince are innovators, I can hear the influence of Sly Stones music in songs like Beat It and 1999. And OutKast's classic song Hey Ya is a sounds like many Sly Stone songs.
I've always views the sound of Sly Stone's music as a hybrid of James Brown, Jimi Hendrix and The Beatles (their psychedelic albums Revlover, Sgt Peppers Lonely Hearts Club Band and Yellow Submarine albums) music.
Sly Stones influence on popular music is in my opinion bigger than James Brown's because of the way he fused different genre of rock and soul together. A lot of people outside the USA don't have a clue who Sly Stone is or know any of his songs, which makes him properly the most over looked artist in music.