Here is the thing that no one seems to have touched on, the only time Tupac was a sort of OKAY at best rapper is when he was a back-up singer for Digital Underground. What made him okay back then was the fact we didn't have to hear/see him very much.
The only thing that Tupac really did for rap was bridge the gap between NWA/Chronic era and moved things into the crap known as rap today. While he had a couple of OKAY songs at best, he just happened to be around when not a whole lot was going on in the rap world.