The worst movie I've ever seen
Many listeners say that movie X is "The worst movie I've ever seen", to which Mark replies it's not and subsequently invites them to his house to see Oversexed Rug Suckers From Mars. The official worst movie ever made. (Although Simon has argued that in his view, it's Dougal And The Blue Cat. Or Pokemon: The First Movie.)
After careful consideration, Colin Murray decided that that Old Horse Face sci-fi feature Knowing was the worst movie he had ever seen. At which point, Mark - yes - invited him round to his house to watch his video collection.
See also Exorcist II: The Heretic.