Boy, that's a tough choice. You have a lot of cardio and cross-training/circuit options. So I'd pick Pyramids and Core Max, so you can work out the whole body using non-cardio workouts.
OR, if you don't mind using Cathe's older workouts, I'd say Cardio Hits for straight cardio (maybe this is on backorder too?) and the PH/MIS/BM dvd for weight work. That gives you six workouts all at once.
Pinky