not sure why jay-z is really an option cuz what did he do for dre outside of "still dre"?
that being said, I had to go with snoop. dre had his best "solo" songs with snoop. I didn't like him rapping eminem's verses, cuz he was doing that multi syllabic flow that dre obviously can't pull off.
2nd best is obviously cube from the NWA days.
also, we should also have DOC and MC ren in the poll. but especially DOC. he's been writing for dre from NWA thru at least chronic 2001, so he's definitely a contender.