Eminem cannot be considered the greatest rapper of all time because of his white skin -- at least according to Dr. Umar Johnson ... who claimed as much on Joe Budden's podcast.

The controversial pan-Africanist -- who's mostly famous for his inflammatory online rants and POVs on race, etc. -- was the latest guest to join Joe and co. ... and one segment from their chat is starting to make the rounds in hip-hop circles, 'cause they touched on Slim Shady.

Watch ... Umar is making a point about whites that are allowed to partake in Black culture -- including hip-hop -- which he says is a disservice to African-Americans ... and doesn't go both ways. In the same breath, he name-drops Em -- saying he enjoys too much privilege.

Joe asks if he considers Eminem to be one of the greatest MCs regardless of race ... which sets Umar off on a tirade about how Caucasians should never be considered to be the greatest in something that he feels belongs to Black people ... in this case, rap music.

Umar even goes so far as to claim that putting at the top of the hip-hop list is "white supremacy" ... but you can tell JB clearly doesn't agree. Pretty wild stuff, indeed.

Gotta wonder what Marshall himself thinks about this ... especially considering his affiliation to Joe and Slaughterhouse. With that said, there have been disses throw out in recent years.

Even with their rocky history, Joe seems to be coming to Em's defense here. Interesting ...