I have to agree with you on Big L and Nas, Big L especially. He woulda been so big if he didn't get shot, especially since he was signing with Roc a Fella but then got shot like 3 days before.
Some others,
Wu Tang
ATCQ (A Tribe Called Quest)
Tupac
Biggie
Logic
Eminem
Lil Wayne (back in tha carter albums only)
Kendrick Lamar
Biggie and Tupac were good but in my opinion, neither of them are the greatest like every says they are. Yes they did make a impact with the few albums they produced before they got shot but I just feel like their death entitled them to that fame since they were "hot" at that time. Both are good but not the best
Slug is definitely a beast as well as eyedea . I think Lupe Fiasco is pretty legit as well. Cunninlynguists makes some pretty good, relax, songs about weed lol. Binary Star is pretty good as well check.out their songs Honest Expression and Reality Check. Can't forget about Jedi Mind Tricks or Army of The Pharaohs