The "must haves" from P90X, IMO, are:
Shoulders and arms (a very effective shoulder/tricep/bicep workout)
Plyo X (plyo--and not the "jump as high as you can" type, but more agility moves and direction changing moves with some high jumps)
Core synergistics (very unique core workout that isn't just the "back and abs" workout most core workouts are. Lots of functional moves.
Ab Ripper X (also included o all of the resistance workouts, so if you get S&A, you get this).
Chest and Back is also good, but is based around push-ups and pull-ups.
I actually like the whole set, because the rotation they set up works very well and is balanced, with sufficient recovery for muscle growth.
For an "ideal" rotation incorporatiing these workouts and Cathe workouts, I would follow the P90X rotation and use Cathe's S&H chest/back (or P90X Chest and back), P90X S&A, PLYO X and Imax2 (alternating doing those each week), Core Synergistics, a yoga workout (Yoga X or another power yoga workout), a kickboxing workout (P90X's Kenpo X is not too intense, but there are some unique arm blocks that aren't in other KB workouts and that were new to me and therefore a challenge), a Cathe lower body workout (either GSL or PLB) (though P90's Legs and back does have some new, effective moves, it wasn't enough for me to keep my booty in the shape I want it to be in! A Firm lower body workout could be used here as well...I find them very booty-friendly!)
In general, I prefer P90X upper body to Cathe upper body, and the sequencing of the exercises and recovery periods have a lot to do with that. I think P90X is much more time-efficient and effective. For example, in S&H, you do a shoulder exercise, then a bicep exercise, then a tricep exercise, then repeat the sequence, then take a break to do dynamic movements and stretches. This gives each muscle group time to recover before it's hit again, but that recovery is active. (I was able to increase weights and or reps on almost every workout with P90X. And got to the point of doing on-the-toe push-ups.)
If Cathe were doing this workout, and following what she usually does, she'd work the shoulders with several sets of several exercises, with a rest between each set, then move on to biceps after the shoulders were exhausted, work them, then move on to triceps. (I really wish we'd see some P90X-like exercise sequences from Cathe.)