Oh gosh, ANY of them would work very well on a treadmill. Volume 8 is ALL hills and it's very hard, but the rest of the workouts incorporate a mix of sprints and hills and/or treks (a fast hill). Volume 2 is my favorite of the shorter workouts and I think you'll be very happy with that one. They're all good and you can't go wrong with any of them . It's impossible for me to name my favorite because that changes with my mood, but I tend to reach for Volumes 4 through 7 the most. You'll definitely want 5 and 7 for the treadmill because they have a faster tempo and are great for running!!I am going to have to use my treadmill for another month or so. I just downloaded Vol 2. What others would be good for a treadmill?