"Real" rappers / Greatest rapper of all time?

I always tell people my favorite rapper is Lupe and people strike back with WTH MAN, have you nevr heard Nas, GZA, Ice Cube, 2Pac, etc???? Well not really actually. I have one 2Pac song. Can someone explain what's so great about them besides being some of the first? Cause as far I know Lupe is on of the most intellectual lyrically intelligent artists, if not the most, I know.