Mid
90's. Althight ironically, that's when people were calling some rappers sellouts or crossover acts.
You had underground cats that bubbled and became mainstream they had success and respect from most ppl (Biggie, Nas, Pac, Snoop, Wu, Onyx)
You had underground cats: Jeru, The Liks, 3-6 Mafia, Common, The Roots, UGK Tribe (close to mainstream)
Straight mainstream: Kris Kross, Fugees (artuably could be in first category)
Notice that u had ppl that earned their stripes from undergeound to mainstream and there's a good regional mix. They developed as an artist before you even heard an album from them so when u heard the album it had a higher chance of sounding polished and good after u already heard their raw ability through opening shows, features and singles.
Cole aan Kendrick also started underground and thas part of why they have staying power.
After the 90's the tales got tipped and the underground became more and more devalued.