How High is definitely the best. Aside from that, Puff Puff Pass and Kevin Spacey in Shrink are solid too—guy probably smoked half the stash in the first five minutes. Saving Private Ryan isn't bad either. Then there’s that documentary about the industry, The Business of Getting High, plus Knocked Up with Seth Rogen. Don't forget Reefer Madness—it's hilarious how stoned and stupid people were back then, though sadly not much has changed. What else... Super High Me, where the guy goes from not smoking for 30 days to being high nonstop for 30 days straight. Wet Hot American Summer. Fat Pizza is freaking hilarious, but you HAVE to watch it while high. My Mom Smokes Weed.
The Rick Simpson Story documentary—definitely worth making mandatory reading in middle school. And yeah, if you want to laugh your head off, I highly recommend Death at a Funeral, specifically the 2007 version. Insane movie. Honestly, I haven't laughed that hard since Fear and Loathing in Las Vegas.
The Rick Simpson Story link:
http://www.vutra.tv/rftc_trss.html