I get what you're saying, but regardless of genre, the vocals are also important instruments. Some rappers were just gifted dope voices.
That said, rappers with the dopest flows and deliveries may be considered good or even great rappers, but they don't enter the realm of being all time greats, because they aren't great wordsmiths or lyricists. If you're not writing your rhymes, you aren't really a lyricist, which would kill any chance of entering the Nas, Pac, Rakim, Biggie, Jay, etc. realm of greatness.