DOOM is my favorite, can't believe how many other people on the thread feel the same way. I love him for his terrible hilarious flow.
I love a lot of old school stuff. Kurtis Blow, Slick Rick, Sugar Hill, Whodini, Afrika Bambaataa, Big L, Wu Tang and what not. More than a few other early things that everyone has forgotten. Gil Scott Heron and the Last Poets, lots of Jamaican stuff that was rap in all but name before rap was a thing, the same for some calypso, proto-rap I guess.
Honorable mention of Mike Ladd for doing things different (why don't Marxist rappers get traction?). I've listened to a lot of Antwon (he isn't the best but something about the way he sees it keeps me coming back).