I'm going against the grain and saying: CTX series. I love separate, individual workouts from each and every series, but CTX is the one series I still get stacks of use from. I have done and do each and every disc and I cannot say that for any other series.
Leaner Legs is still mean, but quick and dirty and gets the job done. The CTX upper split, while endurance lifting, is exhaustive and especially useful if you are coming back to weight training after injury or a break. The real star of the collection remains the cardio workouts. Cathe packs a big punch into each and every one in just about 24-30 minutes each and if you stick 2 of these back to back, these cardios can still give most of her current and recent releases a run for their money. If you love step, these are some of her best routines. Lastly, the music rocks, Cathe is just having fun in this series and it is high energy and infectious. I grin all over my face doing these workouts, no dread whatsoever. Win-win.
Clare