Well its not actually possible to have a judge on whos the best rapper in the world, its whos the best rapper for you. Just old school > New school
for me > 2pac / Method Man / O.D.B / RedMan / Wu-tang clan (Method man is a wu-tang member but i like him both (solo / with a clan) / 50 Cent (few songs, the songs i grow up xD )
And something to add xD a rapper that nobody knows him or some of you might forgot him is Nelly / also grow up with some of his songs xD not in my list but whatever he
HAD a talent.
I also like a lot of greek rappers/groups but you will not understand anyway