I'm a billy blanks fanatic, so I'm pretty biased.
My votes for his best ones: Get ripped Advanced workout 1; ultimate lower body; and most of the early live ones. I found his newer ones (including flex, cardio, boot camp, boot camp elite, cardio circuit I and II) weren't that hard or as much fun. IMO, his Ultimate Lower Body is harder than KPC because there's no let up in the kicking. At the end he has you hold a low "horse stance" for 2 full minutes after you've been doing front kicks, side kicks, round house kicks, back kicks, machine gun round house kicks, lunges to front kicks, squats to side kicks, crescent kicks etc. for 45 minutes with very short recovery periods. (I'm waiting to see what his newest 90 minute one is like. I have my fingers crossed that its a goodie.)
Videofitness.com has very good descriptions of his early workouts which then I bought used from Amazon re-sellers. I have #2, 3, 4, 5,6,7,9 and 12 and love them all. Some have no floorwork, some have a lot. I think all of his live ones are harder than the Kickmax combos, but easier than the blast section which has very few kickboxing drills in it. HTH
Jonahnah
Chocolate IS the answer, regardless of the question.