I would say KPC/LG is the toughest and most fun and effective, lots of useful premixes.
Next up I would say StepBlast and SJP, but I use a lot of premixes on this one.
Next is Timesaver, I use all the upper body segments together, I never use them as short workouts.
I think SS/PP is hard to work with, not very difficult. Some of the premixes are good, and I make a workout using the Mix n Match. It was the biggest disappointment to me of the series.