Finally, Super Size Me.
This wan't my choice at the video store, but I was pleasantly surprised how good it was. As expected, watching the filmmaker get fat and sick from his McDonalds-only diet was scary and disgusting. However, there was a lot more in the documentary that I hadn't expected. This served to add context to his experiment and depth to the film.
Probably everyone should see this, just in case you have any lingering misconceptions that this sort of food might be okay or even not too bad (the filmmaker almost went into liver failure from the diet).