errr.... i cant really name the best, cos all good rappers are the best in their own way if u know what i mean... but i'll give some of the ones that i like the best

nas (poetry)
eminem (multis, originality, went crap recently)
canibus (intellect, wordplay)
redman
jay-z (punchlines)
kool G rap (multis)
big L
aesop rock (wordplay, poetry)
copywrite
immortal technique (meaning)
rakim ( everything)