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.)
See also Exorcist II: The Heretic.