Any or all three of the Bridget Jones films would be fine, though I'd cover at least the first one.

1. The best reason to do it: Have her namesake (Bridget Jones Nelson) be part of the riffing team, if only for all the possible jokes over her name. :)

2. Romcoms -- not enough riffs about those!

3. Can get all meta with the Mark Darcy/Mr Darcy obsession in the film and the fact that both characters were played by Colin Firth.