Nas's debut Illmatic was his first of many classic albums to come in his long and still going veteran career, but what made Illmatic so unique compared to his other albums, classic or not was that he made this particular album at such a young age of 20 and already had the attitude of a much much older rap veteran. Which he definately wasn't yet being Illmatic was his debut and all.
Plus with every track on the album being classic, most noteably Halftime and One Love. Their is no doubt in my mind this is one of the best hip hop albums of all time, if not the best.