His career is basically split into two parts. Pre-Jay-Z beef (Illmatic, It Was Written, I Am..., Nastradamus, Lost Tapes, Stillmatic, God's Son) and post-Jay-Z beef (Streets Disciple, Hip Hop Is Dead, Untitled, Distant Relatives, Life Is Good).
(The reason I split it up like that is b/c he hasn't been warring like he was previously, and post-Streets Disciple, he's had a fairly consistent persona.)
Sure, Illmatic was the epitome of rap at the time it came out, and It Was Written was a damn good album too. But the downward spiral he took by trying to switch directions with I Am... and Nastradamus took a real toll on the perception of his artistic skills.
I think one could make a strong argument that Nas has been much more consistent in his quality of music and the message in his music since Streets Disciple. He's been getting a lot of love from everyone lately b/c of his music, but he hasn't been getting radioplay or attaining mass appeal like he did pre-Streets Disciple. But has the latter-half Nas been the better artistic Nas?