I can only tell you one thing: dont watch Pulp Fiction on shrooms! It sucked more than anything ever and I couldnt stop watching....
The hardest I've laughed while watching a movie baked is when I was helping my GFs sis move into her apartment. She didn't have cable yet and the only movie she had was "Exercising with Diabetes" and it was just a tiny asian women leading all these reallly old people in exercise (they were all sitting in chairs the whole time though, looking miserable or comatose). I probably watched that movie 20 times that weekend and could nooottt stop laughing so, yeah. That's my vote.
Wow, someone recommended Leaving Las Vegas... wasn't expecting to see that one in a thread like this. My recommendations would be, for dramas, Waking Life and 2001: A Space Odyssey, or, for comedies, The Big Lebowski and Super Troopers