The greatest live perfromers ever imo (don't know anything about these new guys) are KRS-One, LL Cool J, Run DMC, and Doug E Fresh.
The mid 90's guys were never that great. Wu could be good (red and meth too). Roots were dope. Jay and Nas never had it in the 90's and were considered quite bad but i hear they've really stepped it up.
Rakim was always pretty bad. Big was dope. He sounded like his records. Slick Rick is really bad as much as it pains me to say it. His dj plays all his records way too fast. Everything is pitched up like +8. He just kind of mumbles and shouts his last line.