To me it's a tie between Frisco & Oakland...
Right now I'd say Frisco, cause I been bumpin mostly classic 90s Frisco shit these days.
RBL, Cold World Hustlers, 11/5, I.M.P., UDI, Dush Tray, Rappin' 4-Tay, Do Or Die, Cellski, Young Ed, GRP, San Quinn, T-Gunn, etc... there was way too many classic albums from Frisco.
Oakland got a grip of em too, from Too $hort to Seagram to Richie Rich to 3XKrazy to Dangerous Dame, Dru Down, Bad N-Fluenz and so on.
Vallejo had a lot of dope rappers, especially for a town that size, but as far as straight up numbers, they still got less dope rappers. E-40's one of my favorite rappers though, the whole Click is tight, Celly Cel, The Mossie, Lil Bruce, the Funkmobb, Mac Dre, Young Lay, Dubee... a lot of talent.