First of the Cheech and Chong movies is probably the best overall, with the duo involved in a plot to drive a van constructed entirely of pot across the Mexican border into the US and featuring Cheech singing "Earache my Eye". Lots of laughs (especially for the "inspired" viewers), but definitely not for everyone. The whole movie revolves around drug use, so not a movie for young children, obviously. However, if you don't mind this, you'll probably laugh your you-know-what off.
Review of Up in Smoke
