Public Enemy is one of my favourite bands. Some other notable artists, in rough chronological order of their best material: Eric B. & Rakim, Ultramagnetic MCs, De La Soul, Beastie Boys, Gang Starr, Cypress Hill, Freestyle Fellowship, Wu-Tang Clan, OutKast, Organized Konfusion, The Roots, Eminem, cLOUDDEAD, Dälek, El-P/Company Flow
Somewhere along the mid-90s a lot of rap music became a little bit too repetitive for me - it always was to begin with, but the dynamics totally went out the window at some point when people settled on one sample for an entire song and mostly left it at that. A good example of this is how DJ Premier went from his earlier style with the first couple of Gang Starr albums to The Sun Rises in the East which has these great samples but looped incessantly - just a little bit more melodic progression would go a long way. And of course it's one of the most critically-acclaimed hip-hop albums of all time. It marks a point where I stop understanding a lot of people when they profess albums in the hip-hop canon to be "classics", and with the exception of Eminem my favourite hip-hop artists past that point in the mid-90s tend to be alternative rap of one kind or another.