I'd think the #1 would have to be California State-Humboldt. Humboldt County is like the capital of outdoor Marijuana growing in the U.S. and one of the big draws for Humboldt Sate is their Botany program. It doesn't take a rocket scientist to figure that one out.

UC Santa Cruz would also have to be pretty high on the list. Everyone I know who's gone there was a toker. The campus is tucked up into the Santa Cruz mountains and they routinely find pretty large patches of weed growing on the campus itself.