Biggie, Tupac, Andre 3000, Nas, Rakim
Why does everybody sleep on Andre? My man can completely lose you in two bars. If you're not paying attention, you'll think his bars don't make sense. Then you listen to the two bars again, and you realize his wordplay is just on another level.
EDIT 2: Honorable mentions; Jay-Z, Eminem, Common, Talib Kweli, Mos Def.....yo, it's really hard to narrow this down! F it!!