Toxic Avenger rule
A triology in which the second is better than the first and the third is better than the second. Established after a listener wrote in to ask if there was any film sequence that followed this pattern. (There are several series in which the second is better than the first - the Godfather, Star Trek, Terminator and Alien franchises amongst them - but they are all let down by the third one). It was suggested that the Bourne films - which at that point were still a trilogy - would qualify, but Mark Kermode argued that Supremacy is every bit as good as Ultimatum, and so this was discounted.
The only trilogy that could be found to fit the bill, therefore, was the Toxic Avenger sequence. Mark did, however, add that they are all so bad that the third is still staggeringly awful, just not quite as bad as the second, which was still fractionally better than the first.