This thread is more than truth.
Stillmatic, Life Is Good, God's Son, and Untitled are all damn near perfect "classic" material. Hip Hop Is Dead is tremendous, Distant Relatives is tremendous. Streets Disciple is good, could've been great if cut down to one disc... that's really his only misstep post 2000.
And if you consider The Lost Tapes post-Stillmatic (as it is technically) then that just adds to it. If you consider The Lost Tapes pre-Stillmatic since that's when those songs were created, then it makes it a tougher decision because you have Illmatic, It Was Written, and The Lost Tapes, plus I Am had some classic cuts and NAStradamus is really only weak compared to the rest of Nas's own catalogue, not weak compared to the rest of rap.
As much shyt as people try to give him, Nas has been a consistently great lyricist throughout his career. He's never actually fell off. I'm not sure who else I can think of in the rap world who has continually topped himself.