Poultrygeist. It's a Troma production so it's revels in its B-movie nature and the self-awareness makes the script ingenious. It is also an example of how to be hilariously non-PC while not being seriously insulting towards the targets. An example:
One of the main characters is a Muslim woman wearing a niqab so for almost all the movie we see only her eyes. Nobody pays attention to this or treats her in any way differently; she is just another normal character. In the end, when the heroes are about to get caught by the undead horde and have no escape she makes the ultimate sacrifice to save everyone: she removes her niqab revealing a hot woman wearing only a bikini and a bomb belt, lets out a Mid-Eastern style scream and rushes at the horde before blowing herself and the undead up.
A real so bad it's good movie: Samurai Cop. An eighties action flick where not only the acting and script but also the post-production is so bad that it's hilarious.