As of now Hit-Boy. There’s not ONE bad song across the 6 straight albums they did. Even the loosies and one-offs they did were fire. Hit-Boy got Nas to reunite The Firm, reunite with Premier, do a song with Wayne, and probably most impressively, get Lauryn muthafukking Hill to come out of hiding.
Secondly i’d say Salaam Remi. Their catalogue together is insane. People immediately think about Made You Look but there’s also What Goes Around, I Can, Get Down, Bridging The Gap, Thief’s Theme, Where Ya’ll At, Da Don, Bye Baby, Nasty, etc. Salaam was consistently getting the best out of Nas for damn near 12 years.
Honorable Mention Damien Marley. He absolutely LACED Distant Relatives. Nas has said that album helped him get through the divorce from Kelis and the custody battle and you can listen to songs like Strong Will Continue and tell that Nas was channeling a lot of those emotions and feelings into the music.