Toughie...you should be more specific. Every rapper has their own specialist strengths.
(remember, this is all just my opinion)
Potentially, the best rappers of all time are Nas and Canibus, but both have made just one good record each, and are consistently disappointing.
So in my humble opinion, the greatest rapper of all time would be one of these 5, as I believe them to have the best all round abilities:
Deacon The Villian
Slug
Immortal Technique
Aesop Rock
Ice Cube
but seriously, my knowledge of hip-hop is pretty limited. for all i know I could be way off 😬 I certainly haven't heard enough of Immortal Technique to put him up there really.