The Gothic series had a lot of drug use. "Swampweed" actually played pretty prominently in the first game, with an entire camp of stoners. The Elder Scrolls had "skooma" which was an illegal beverage, but considering the availability of various alcohols, one must assume that skooma is a drug.
If I had to choose a game that would be most fun to play while stoned, I'd say Skyrim. Huge environments to explore and not terribly difficult once you get your character up a few levels. Epic vistas, a lot of wandering around and plenty of inane conversations with NPC's to entertain you.