Best rapper ever is Tupac Shakur - no question about it.
Best rapper alive is Jay-Z. Followed by Eminem, Nas and various members of the Wu-Tang Clan - Raekwon, GZA, Method Man, RZA.
Best rapper as of right now is Lupe Fiasco. He's bringing a new style and his words have meaning. Not the boring trash you hear coming out of 95% of rappers mouths these days.