He's my GOAT, personally, but obviously other people got other opinions.
I look at it like this:
-Lyrics
-Flow
-Discography (ie albums)
-Songs
-Personality (example: charisma)
Not every factor is equal. For instance Nas has often had a rather low key persona, he wasn't really out there promoting himself like a Jay or Pac. But obviously personality isn't as important as lyrics or albums.
Lyrics: over his entire career, I give it to Nas. Some rappers have challenged his sheer lyrical mastery, like Common and Mos Def, but neither were consistent enough throughout their entire careers. To me, consistency isn't a single factor, it relates to everything on the list except personality.
Flow: This should garner lots of debate. Some will agree with Jay's argument on Takeover, that Nas has a one dimensional flow that rarely switches up, and can't really work on certain tracks. There certainly are examples of him trying a new flow and failing...
I'd simply argue Nas perfected the best flow: that Queens multi layered, multi syllable flow. The G Rap flow. Is he as diverse as Jay-Z with the flows? No. But give me that Queens shyt over anything else.
Discography: Three classics: Illmatic, It Was Written, Lost Tapes. Two are the best of their respected types of rap, IMO: Illmatic is the best rap album ever IMO, and IWW is the best mafioso record. Three great albums: Stillmatic, God's Son, Life Is Good. His other albums range from solid (Untitled) to bad (Nastradamus). Not sure I can think of anyone with two better albums than Illmatic and It Was Written...
Song: I give this to Pac and Jay-Z. Both don't really have albums that compare with Nas' best, but overall they have a shyt ton of classic songs. Great beats, great hooks, dope features, etc. Nas has a bunch of classic songs too obviously, but I'd still give that crown to Pac and Jay...
Personality: I dig Nas' laid back personality. He had the world in his palms and still never really got gassed up. Obviously someone like Pac would be #1, but Nas is up there.