Rap
Neptunes(the greatest... They ran popular music for 5 years... The only record they didn't have was a Gospel and Country track.)
Timbaland(Single-handedly saved R&B from dying in the mid to late 90's and created signature sound that helped lead southern music into mainstream)
Mannie Fresh(The second greatest one man genius to ever produce)
Dr. Dre(Brought Rap into modern era.... Both Chronic albums are masterpieces.)
Premier(Diamond standard for the east coast sound. Style is copied studied all over the world)
Honorable Mentions: Large Professor, Pete Rock, Mike Dean(Rap-a-Lot), DJ Toomp
Modern R&B
Kenny "Babyface" Edmonds
Jimmy Jam & Terry Lewis
R. Kelly
Teddy Riley
Keith Sweat
Honorable Mention: DeVante Swing(Damn drugs...

)Raphael Saadiq
Other
Quincy Jones
Rick Rubin
Berry Gordy/Smokey Robinson(Motown Sound)
Prince
George Clinton