I would say it really depends on what you define as "Rap."

And there are many different sub-genres of Rap, just as there are different sub-genres of Rock.

There's your classic retro 80s Rap like LL Cool J, the Beastie Boys, Run DMC, etc.

There's G-Funk like Dr. Dre, Snoop Dog, Warren G & Nate Dogg.

There's Gangsta Rap like 2pac & Biggie.

There's "cerebral" Rap like Wu Tang Clan, Grave Diggaz, Jedi Mind Tricks, Company Flow.

There's "Horror Rap" like Insane Clown Posse (who owe much of their style to Esham, as does early Eminem -- the dude literally says in a track "I'm a cross between Manson, Esham & Ozzy").

There's dark abstract rap like Cage (Chris Palko) -- and his later albums like "Depart From Me" & "Kill The Architect" dabble in an Industrial sound that's kinda reminiscent of bands like Skinny Puppy & OhGr.

There's "Conscious Rap" like Common, Talib Kweli & Mos Def.... Also, Sage Francis & B. Dolan.

There's super underground stuff like Eyedea, who made tracks that were practically art. Like "Music Music" and "Birth of A Fish" and "Here For You."

There's Rock Rap like Kid Rock (he ended up going fully into Country).

There's also purely Instrumental stuff like The Avalanches, Nujabes, Vanilla.

There's "Melodic Singing Rap" like Bone Thugs N Harmony.

Rap / Hip Hop was diverse ~ even before it was fully mainstream.

I would that the internet provided an opportunity for not only different genres in rap to grow but also become mainstream. A good reference to this is OPs metal - the genres within metal are vast and have grown over the years from sabbath/thrash/black into a hundred genres all more nuanced. Hip-hop and rap are similar and it allows for more individuals to find a niche they like.