Whenever I've taken on Hi-Low Intervals from the Hardcore Extreme DVD, I've believed I might die , so this one is THE most challenging for me. I also really struggle with To The Max.
I think the Plyos from STS are quite the feat weights wise as is Xtrain Bi's+Tri's & Muscle Endurance. At least it is for me.
Tabatacise, X10, and Cardio Core Circuit. These have the most dread factor for me, so I think of them as the most challenging.
As far as the most challenging weight workout, I would vote for Ramped Up Upper Body. It is my FAVORITE upper body workout, but I always forget how difficult it is. Just when I think I am done with a body part, there is one more exercise, and one more exercise, and yet another exercise! Each and every muscle in the upper body is beaten into absolute submission. Hurts so good!
Most recently, Strong & Sweaty Cardio Slam. My HR was up there. Of early Cathe's, Imax 1, Drill Max, and in rotation, High Step Challenge & Step Blast.