I can't tell you about Rakim, Gang Starr, Cormega, Souls of Mischief, Little Brother, The Roots. But I can tell you about Nas and Lupe Fiasco.
I don't like the rap genre except for a few artistes. Those are 2Pac, Nas, Kanye West and Lupe Fiasco. 2Pac is my favourite and IMO, is definitely the best. In fact, he is the reason I started listening to music, let alone rap.
Nas is a great rapper, but these days he is getting boring. I can't say about his last 2 albums but his flow has definitely changed for the lamer, and his lyrics are kind of boring now. Illmatic is even with All Eyez On Me as my favourite album ever. He was young, hungry and had the best flow I ever heard. Later on, he started adding more depth to his tracks. But from God Son I think he has declined. He still has great recent songs, but a lot just sound like CD fillers.
Lupe Fiasco is my favourite current artiste now. Apart from his flow, and his witty lyrics and metaphors, he has some deep songs as well. However, he has some songs from earlier on where he was just a rapper with a tight flow but crappy material, but that has changed. I don't think he can be compared to Pac though. completely different, 2Pac was less of a rapper but more of a lyricist. He beats Lupe for depth, Lupe beats him for flow and style. Pac also had more charisma and more pasion, Lupe has more energy. I think he can become a legend though, along with Hurricane Chris (joking)