Circuit Max, on the same DVD as Cardio Kicks (a kickbox-flavored floor cardio workout). Circuit Max is the one I come back to at least once every two-three weeks.
Also, the Gauntlet on Cathe's Terminator DVD (a compilation disc of 3 workouts, with combined footage from several of her cardio and weight workouts from the Intensity Series), is awesome, and because it is so finely chaptered you can tailor it to what sort of circuit style you want for each workout.
A-Jock