Not all movies are serious or straight-forward. Some take hilarity to new levels by being completely wacky, and if it doesn't make sense, too bad. The following 6 movies all fall under that category.
The Inspector General can be best described as Danny Kay being silly. Don't let its age or the fact that it's a musical deceive you -- you will definitely laugh.
This Mel Brooks' film parodies the dark and serious Frankenstein story. From the first scene, it loses seriousness, and goes even farther from there on.
Bill and Ted's Excellent Adventure
Keanu Reeves, Alex Winter play two dumb high school students who use a time machine to pass their history class. You'll never see The Matrix the same again after seeing Keanu Reeves play this role.
Tenacious D -- The Pick of Destiny
The crazy band Tenacious D decided to make a completely fictitious movie about how they came to be. Watch it more as a selection of connected music videos rather than a well-made movie.
Another Mel Brooks' film, this time without any speaking parts. It really is a silent movie, made long after they became outdated. Even stranger, the plot is about three people trying to make a silent movie after they have become outdated.
Nothing beats John Belushi and Dan Aykroyd being the epitome of cool in The Blues Brothers. Actors playing musicians and musicians playing actors, this movie turned out perfect somehow.