Since creating my post on the Amen Break I've been looking for a definitive ranked list of the most popular samples in music. I haven't found that yet but I did come across the-breaks. An extremely comprehensive site that lists the source of samples and the tracks they have bee used in organized by genre. After The Amen Break it looks like James Brown is the most sampled artist.
I just found this site, Second Hand Songs, which contains a list of most sampled performers. Though I question the accuracy. No Winstons (Amen Brother)?