The truth is that Prime RJJ is the GOAT of all of combat sports. There is no MMA RJJ and the sport will need to be around much longer to esatablish one. Prime RJJ is the GOAT of a sport that established itself for hundreds of years.
He went nearly 50 fights without losing a single round, schooling the likes of greats like James Toney and Bernard Hopkins. Dominating, clowning and destroying his opponents so badly they didn't even look like they belonged in the same sport from middleweight all the up to heavyweight.
There are other fighters with arguably better "resumes" and "accomplishments", but nobody else was as gifted and dominant in their prime.
If you took all the greatest fighters in boxing history and you somehow made them all the same size, prime Roy would blow through all of them.