This is a tough call. I think if you go as heavy as you can during PP, PUB, S&H, GS, MIS's upper body section, or PS - then any of these could be viewed as the 'toughest'. Okay, this is what I'd say:
1. Chest - Gym Style (I agree!)
2. Back - Pure Strength (going as heavy as possible) or CTX
3. Shoulders - PUB (using 8-10-12-10-8, or whatever challenging weights work for you)
4. Biceps - Gym Style or PS for strength (I think CTX is the TOUGHEST in terms of endurance, tougher than ME or PUB); for strength, I find GS to be tough as there are so many excercises available (and Cathe uses 20lbs for concentration curls!) and in PS's biceps section, Cathe starts with a 45lb. barbell!
5. Triceps - GS is tough for me. (CTX is tough in terms of endurance.) For me, the toughest tricep excercise there is is tricep pushups. I also think skull crushers (or lying barbell extensions) are very tough, so for me, Power Hour's tricep section is a doozy.
I think it all depends on how heavy you go. I can get a great upperbody workout with PP or SS simply by going as heavy as possible. I love to use all 5 CTX sections back-to-back as a strength workout (instead of the intended endurance workout). For Chest, I use a 45 or 50lb barbell and go slower than Cathe. I use heavier dumbells and instead of doing the set of dumbell presses that she does, I do two sets of flies. For Back, I use a 45lb barbell for the rows and take longer rests between the sets. For the one arm row drop sets, instead of using 20,15, and 10, I use 25,20, and 15. For shoulders, I use 12lbs during the giant set of side laterals, rear flies, and front raises. For biceps, I pretty much use what Cathe uses but I do try to go heavier on the alternating rotation curls and the concentration curls (if I can). For triceps, I go heavier for the lying extensions and I like to 12lbs for the superset of one-arm overhead presses and kickbacks. Again, it's all based on how heavy you can go, and what your goals are. Somedays I like to do endurance, but I still try to go as heavy as I can while keeping up with Cathe's speed and maintaining proper form.
