Rap/hip-hop is a genre that is constantly being redefined -- often deconstructed and reconstructed. As such, "best" is very relative. It's kind of like asking what jazz album you like best. Well, it depends on what style you like.
There are lots of artists who have (re)defined the genre over the years -- esp. in the mainstream.
Honestly, Missy Elliot has probably done more to transform hip-hop over the past ten years than anyone.
Other notables who have had a significant impact on the genre via performing and producing:
Grand Master Flash
Run DMC
Public Enemy
Dr. Dre
Notorious B.I.G.
Wu Tang Clan
Whatever Sean Combs calls himself these days
Eminem
Fatboy Slim (does he count as rap/hip-hop?)
... and, yes, even the Beastie Boys.
I feel bad because I know that I'm leaving some artists out, but these are the ones that my drunk ass can remember at the moment.
Anyway, I'm listening to Rip Slyme now.
mvp