Circuit Max, closely followed by Gauntlet from the Terminator DVD. No muscle group is left unburned (although Gauntlet does not have any core work), and the cueing, music and choreography make the time fly by on each of these.
The fun factor, I might add, in each of these is matched only by the sweat-where's-my-water-bottle factor. These are among the top for challenging workouts, especially for cardio.
A-Jock