Plan 9 is the ultimate example of So Bad It's Hilarious.

I laughed 'til I hurt the first time I saw it; it's had the same effect ever since. Who couldn't enjoy lines like these:
Criswell: "And remember, my friends, future events such as these will affect you in the future."
Air force officer: "Visits? That would indicate visitors."
Wife: "I've never seen you in this mood before."
Husband: "I guess that's because I've never been in this mood before."
Wife: "Now toddle off and fly your flying machine."
Army colonel: "For a time we tried to contact them by radio but no response. Then they attacked a town, a small town I'll admit, but nevertheless a town of people - people who died."
General: "Atmospheric conditions in outer space often interfere with transmitting."
Criswell: "My friend, you have seen this incident, based on sworn testimony. Can you prove that it didn't happen?"
Priceless.
*** leaves to find and watch it ***