5. Curtis Mayfield
4. Zapp & Roger Troutman
3. The Isley Bros.
2. Stevie Wonder
1. James Brown
James Brown first and foremost because on top of the immensity of his songs, he was also the first true funk artists, at least as far as I know.
Stevie goes without saying for my tastes. Songs like "Do Yourself a Favor", "Jesus Children of America" and "Maybe Your Baby" are just killer, killer funk tracks, and he also fused some of the funkiness into songs like "Tuesday Heartbreak".
Isley's have some amazing funk tracks. I love the mellow funk of "Showdown" and the deep funk of "Fight The Power".
Roger Troutman goes without saying as well.
Curtis Mayfield also had the mellow funk and the funky-funk down pat. Even on hits like "Let's Do It Again" you can't escape the funk, just listen to the guitar riffs at the beginning.