i agree with luminous.... all good rappers deserve to be in the top 3 for one reason or another... it all depends on taste.... if you think lyrics are the most important, then canibus would be 1st for you... if meaning is most important, than 2pac will be there... if poetry is big for you, Nas will be your 1st choice... but on the overall, all those things put together, jay z nas and eminem have the first 3 positions from the rappers that are still alive... dont get me wrong, canibus, talib kweli, common, DMX are all great rappers and have their own special thing which no one can do as well as them, but on the overall, they lose to jay z nas and eminem... ill break it down:

for example, eminem vs canibus

Flow: Canibus
Lyrics: Canibus
Meaning: Eminem
Overall "dopeness" of albums: Eminem

You gotta admit, eminem's skill in his battle days (infinite days) were comparable to canibus except for the fact that canibus is all scientifical etc.. but anyway, in pure potential, the two are very similar (canibus just beats him by a little), but in meanings and quality of albums, eminem wins by miles, besides rip the jacker, all of canibus' albums had wack beats, and 90% of his songs are made up of the battle topic, while eminem talks about life and things people can relate to...

so this proves that some rappers might be the best in one category, but overall they are overshadowed by others... so while Talib might at times be better than eminem, nas or jay z, and might have deeper songs, on the whole these 3 ^ are better