A lot of people are picking artist' who might pick a contemporary beat which may seem "hot" at the time but has no longevity.
What I judge it on is how dope it is and the longevity of the song, can I listen in years time?
Pac might be my pick cause he made so much classic music and that's what it's about.
Biggie had some great beats, Cormega on certain projects had some excellent beats, AZ here and there, Guru with Gangstarr of course, K-Rino has a lot of beats I love, KRS-One, Atmosphere with Ant, a certain era Mobb Deep, Jay Z has rapper over some really dope beats.
I think Jay isn't a bad answer in a sense when you consider his whole discography. Fat Joe can certainly pick some good beats, Cam did for a while.
To me it's an impossible question, at a push I'd say Pac though I think....