Good question.
I’d have to say Nas & Salaam for singles. Nas & Hit Boy for an entire ALBUM.
Salaam has given Nas some of his greatest individual songs. What Goes Around, Made You Look, I Can, Get Down, Bridging The Gap, Thiefs Theme, etc etc.
He also co-anchored Life Is Good, one of Nas’s later day classics.
But Hit Boy can give Nas a Salaam type sound. He can give Nas a Dr Dre type sound. He can give Nas a Kanye type sound. Hit Boy can do it all and thats proven to be IMMENSELY helpful to Nas in crafting an entire project.