Up in Smoke, a seminal entry in the "stoner film" genre, chronicles the adventures of "Man" (Tommy Chong) and Pedro (Richard "The Cheech" Marin) as they drive a car made of hardened marijuana across the Mexican border. Other things also happen.
Up in Smoke is important because it establishes some very strict "stoner movie" patterns that every movie on this list will follow closely: We have two lead male characters on a journey with a loose (if defined) goal, who encounter multiple bit characters and an antagonist that is only tangentially (if at all) related to the main plot. Remember this part, because it'll get important later.
The movie opens with the clearly 40-year-old Tommy Chong being told by his parents that he needs to get a job or he's going to military school, so let me just say that I had trouble suspending my disbelief and was not immediately hooked. But, I gotta say, the movie eventually won me over through the freaking expert-level delivery and physical comedy: Cheech and Chong's characters have become archetypes, and you can see them establishing real motifs here. Most modern imitators: Eat your frickin' heart out, because these guys are just, like, fucking hilarious, man. Stand-out scene is when a motorcycle cop pulls over their weed car. The exhaust is infused with THC, so the cop becomes instantly high, and when he approaches the window he looks Chong nervously in the eye and stammers, "... What do you guys want?"