Leaving it at emcees and rappers, all things considered and thrown in equally, I maintain that Mele Mel is the best emcee the culture has produced. But he's not my favorite.
It's sort of childish and flimsy to think "my favorite ____ is the best ever" because usually it's based on personal perspective. Contrary to popular belief, you actually can quantify the technical aspects of the art if you had to, when we don't allow for it to be discussed because of personal feelings/contemporary bias, it can get old quickly.