I think the Intensity Series is Cathe's best yet. I have kept ALL of the DVD's from that one (except the compilation one). PUB/PLB is a fantastic weight workout (with some easy-to-do modifications, like more rest between sets on the leg work), IMAX2 is fun and effective (and less high impact than IMAX3), Bootcamp and ME are great workouts as well. There's great variety.
There are fewer workouts in the Intensity Series, but they are all keepers, IMO.
For me, the Hardcore Series had some great workouts (Gym Styles, Low Max), some that are o.k., but not my favorites (IMAX3, but with lots of modfication!), some that didn't appeal to me at all (Stretch Max, Kick Max), and some I haven't made up my mind about yet, but which I don't really gravitate to (High Step).