The reason I think Nas is the greatest IMO is on 3 things
1. Lyricism
-His compound rhymes within a rhyme is too good. His words leave you wondering what he really meant in a deeper sense
2. Concepts
-Although he's not the first to use conceptualism in his raps, he is the innovator and took it to another level with his philosophies and storytelling. Influenced Lupe a whole lot
-Illmatic album was a street hustler theme
-It Was Written was a mafioso style theme
-Streets Disciple was a biblical theme
-Hip Hop is Dead is a theme of old musical classics
-Untitled was a racism in America theme
-Plus countless other individual songs that have awesome concepts
3. Can rap in any style
-He can rap fast, slow
Can rap on southern hip hop beats
http://www.youtube.com/watch?v=E9iT_fUCVf8Can rap west coast hip hop beats
http://www.youtube.com/watch?v=rqqrr-ZiNO4 Raps about World history and teaching the youth
http://www.youtube.com/watch?v=zLmRoVMfsD8Relationships
http://www.youtube.com/watch?v=LmZ5Muqyx8sCan personify objects into a story
http://www.youtube.com/watch?v=eUwJ8WcZ6RYOthers may have their favorites, but Nas to me is the greatest and most influential