I think the whole set is the best fitness investment I've made for a long time.
If I had to choose two,it would be tough, but Plyo X (because it's different from many other workouts, and the Plyo isn't all high jumping in the air, but very functional directional changes, etc, without torquey moves) and Core Synergistics (also because ít's an interesting functional workout).
But Shoulders/arms is SUCH a great weight workout (even if you only do 1/2 of it), it's hard not to go with that. You could alternate it with Cathe's Slow and Heavy Chest/Back if you don't want to get P90X Chest and Back (lots of push-ups and pull-ups..the most intimidating of the P90X workouts, IMO).