You have to start no later than 1986 with Rakim to provide the proper backdrop. Once you're actually into the 1990s, it's not nearly as important to fuss over whether you pick "Mo' Money Mo' Problems" or "Victory" or "Hypnotize" to showcase the world of glossy late-1990s rap, for instance.
But if you want a textbook for the class, Shea Serrano's
The Rap Year Book would be a place to start.