Bit laughable having some 'franchises' in there that only actually have two full games, such as Bioshock, Dragon Age and Starcraft, although I suppose you could include expansion packs like Awakening, Broodwar etc.
One thing that's a bit difficult to judge as well is when you have massive franchises (lets say 5 or more titles) released over many years, when we talk about the franchise getting better or worse are we purely talking based on the most recent game (easiest to define) or some sort of broader measure of quality over a number of titles? For example, I much preferred COD:MW3 to COD:BO, meaning it is 'improving', but then I also preferred some earlier games in the series over MW3....