I think it depends on your goal, exercise preference, and type of workout:
Favorite non-interval step: Step Blast
Favorite interval step: Imax 2
Favorite bootcamp style: Bootcamp on 4DS; Drill Max is 2nd
Favorite upper body weight work: Gym Styles
Favorite lower body: Butts and Guts
Favorite circuit workout: the Terminator (Viper or Gauntlet)
Favorite high rep weight work: Muscle Endurance
Favorite compilation: the Terminator
Favorite kickboxing: Kick Punch and Crunch
I have all of Cathe's DVDs except Pure Stength and Ab Hits. The workouts I do most are Gym Styles, Butts & Guts, Drill Max, Imax 3 (I like Imax 2 better, but 3 is more of a challenge), 4DS, and the Terminator workouts.