1. Big Lebowski (nothing more to be said, really)
2. Cheech & Chong's Next Movie (by far, the best of the C&C films, if you want a truly surreal experience, though, the bowdlerized version they showed on broadcast Fox has to be one of the top ones. They managed to elide all mentions of drug use from a Cheech & Chong film, and somehow transformed the huge bag of pot that Cheech's cousin Red was carrying from pot into diamonds. The sheer audacity and stupidity of creating a version of a Cheech & Chong film that has somehow magically become a non-druggie film just boggles the mind. I doubt you could find this version anymore, I'm sure the DVD version is the original, and I doubt Fox, or anyone else would ever broadcast it again, so, if you saw it, you know exactly what I'm talking about, and if you haven't seen it, then you don't know what you're missing)
3. Dazed and Confused (More about the end of high school than about getting high, and Rickey mentions two people who really didn't have big roles in the film, where the biggest star of the film was clearly Abraham Lincoln head)
4. Friday (seriously, any stoner comedy list that doesn't have this classic included, is very, very misguided, or at least very, very white)
5. Harold & Kumar Go to White Castle (More about the American Dream, than about getting high, but the NPH stuff saves this from being stupid and lame, instead of stupid and awesome)
6. Zardoz (I know you may object to calling this a stoner comedy, but seriously, this film is hilarious, and everyone in it had to have been high as a kite while making it, plus Sean Connery in big red diapers)
7. Topper (Alcohol is a drug, dammit, and this film is damn funny, and seeing ghosts seems pretty trippy to me, so I'm calling this a stoner comedy, and if you don't like it, tough)
8. Pulp Fiction (The first and only 'heroin comedy' ever) UPDATE: Bill in the comments reminds me that Pulp Fiction isn't the first or only 'heroin comedy' as Liquid Sky exists (I just didn't remember it, and when I did remember it, remembered not liking it much), also this review makes a case for Trainspotting being a 'heroin comedy', but I'd classify it more as a drama with occasional comic and absurdist elements.
9. Head (The Monkees film, this film is trippier than it has any right to be, and despite being as dated as hell, has some pretty absurd moments that approach being actually kind of funny)
10. Pineapple Express (This film tentatively holds a space in the top ten, Franco and Rogen together again seems like brilliant casting, and I don't see how this film could possibly go wrong). Turns out it does deserve a spot here, it's a pleasingly goofy, yet strangely violent picture that brings together a lot of talent for a lot of silliness. Franco and Rogen really should do a comedy together on an annual basis, a la Hope/Crosby or Abbott/Costello.
Bonus Pick #11 Smiley Face (that's right, this list goes to 11, and even though the characters in Spinal Tap are likely drug-addled, I'm not counting that as a 'druggie comedy') Anna Faris nails the previously overlooked, hot chick who happens to be a stoner, stereotype that too few comedies explore. She's fantastic in this film, and her misadventure is entertaining.
So, that's my lazy half-assed attempt at acknowledging this ridiculous 4/20 crap. Enjoy.