As someone who doesn't believe in the "Nas picks bad beats" narrative, let me play devil's advocate.
Illmatic was Nas' first album and he was working with an all-star team of producers, all of them in their prime. It was a combination that was unheard of at the time, but it also meant that unless something horrible went down during recording, the album would have amazing beats. Premier, Q-Tip, Pete Rock, Large Professor, and L.E.S. all working on the same album in 1994 with the greatest MC of all-time? There's no way it could fail. That kind of album had been done before (Down with the King by Run-D.M.C.) but Nas was different. There was no way he would get average production from any of these guys. Premier also sequenced the album along with working on it.
As time went on, Nas had more control over his music and the beat wasn't a concern as much as the overall song. It wasn't about the hottest beat, it was about which beat would fit the narrative he was going for. He would also work with a lot more producers, and depending on how much time he spent with them, he wouldn't have a chance to get more than decent work from them.