Funny, I just did R1000 again this morning, and I agree, it's a killer. My legs were fried by the time I was done!
I have some Cathes that truthfully I'm afraid to try, and I have no desire to even acquire Imax 3 as I've heard it borders on the impossible! I traded away her Terminator without even having tried it!
I guess I can't decide which w/o is the most challenging as it depends on my mood. Some days the very thought of doing a certain w/o makes me shudder and other days I crave it.
I will say that as much as I love Bryan Kest's Power Yoga series, the second session, Tone, is very difficult for me. It's very challenging physically, and for me, it's challenging on a mental level too, as are some of his live class audio downloads. I have to be in
just the right mood to take on those.
ETA: Laura's post reminded me that P90X Chest & Back probably is my most challenging w/o simply for the fact that I dread it so much. It is nothing short of a push up/pull up/chin up marathon. Three excercises I'm not a fan of!