Well, let's see. That's tough. My current favorite is his new one, Vol. 7. Even though it's targeted more for treadmills, it works really well on my elliptical. I love the first challenge, which is really long and pretty tough... but more of a steady state than any sprint work. And I really love the music in that challenge.
Vol 6 with Sean runs pretty close though, and I think all of the challenges really flow well. Overall, it seems the smoothest.
Vol 6 with Candace... that's a kick-butt workout for me. She does tons of level 4 sprints, and sprints just kill me. But I really like her as an instructor.
Press Play always seems to give me the biggest calorie burn. It's very well put together, but the real reason I like it is the reason Coach Sean made it, and because of our CC PP Saturdays here.