I'd have to say:
Body Max 2's Boot Camp premix
The Gauntlet
Drill Max
High Step Training Advanced
The first workout on the Hardcore Extreme DVD is more cardio based, but I find it quite challenging.
The 85 min. premix from SJ&P and Step Blast
If you put your all into the cardio, I find Low Impact Circuit to be a nice challenge - those triceps pushups and the biceps curls with the bar and band really get me.
Another thing to do (if you have a multi disc DVD player) is to make your own circuit. Put in a cardio DVD, then a total body DVD. For instance, I made my own upper body circuit using Low Max and Muscle Max. I put Low Max in then put Muscle Max in setting it on the upper body only premix and skipping the warm up so that it started at the pushups segment. It went like this:
Low Max warm up
Low Max cardio/blast 1 -- hit 'disc skip' on DVD remote
Muscle Max chest -- hit 'disc skip' on DVD remote
Low Max cardio/blast 2 -- hit 'disc skip' on DVD remote
Muscle Max back -- hit 'disc skip' on DVD remote
Low Max cardio/blast 3 -- hit 'disc skip' on DVD remote
Muscle Max shoulders -- hit 'disc skip' on DVD remote
Low Max cardio/blast 4 -- hit 'disc skip' on DVD remote
Muscle Max biceps -- hit 'disc skip' on DVD remote
Low Max cardio/blast 5 -- hit 'disc skip' on DVD remote
Muscle Max triceps -- hit 'disc skip' on DVD remote
Low Max cardio/blast 6 and 7 -- hit 'disc skip' on DVD remote
Muscle Max - go back to main menu and do abs and stretch
It works perfectly because both Low Max and Muscle Max use 8" steps so you don't have to run and set up a high step, etc...
You can do this in gobs of combinations with dozens of DVDs.
I like to do this with any of the Imaxes and a lower body workout. For example, throw in Imax 2 and Pyramid Lower Body. Set Pyramid Lower Body to the lunges segment (then you won't have to try to set up a high step as you're already using a club length step).
Imax 2 warm up and cardio/blast 1 and 2
PLB lunges
Imax 2 cardio/blast 3 and 4
PLB squats
Imax 2 cardio/blast 5 and 6
PLB dead lifts
Imax 2 cardio/blast 7 and 8
PLB calf raises
Imax 2 cardio/blast 9 and 10
PLB floor work
Stretch