CK might be most like CTX kickbox, as they were produced around the same time. CK has an ending "drills" segment (kicking and punching). The workout feels more "kickbox flavored cardio" than KPC at times (and some moves use a lot of space if you do them "as-is": I think 16' front-to-back and 20' side-to-side).
KPC is more "pure kickboxing" than CK.
Both are good workouts, but I (who love more 'authentic"-feeling kickboxing workouts, liked KPC right away, and had to warm up to CK.
I personally didn't like KM; it was a real disappointment to me after KPC. The beginning kickboxing moves are lower intensity than KPC, and many of them have pivots (which seem to take the intensity down). The second segment of "jumping around like a cheerleader" drills didn't feel too "kickboxy" to me most of the time, but is great if you like tuck jumps, plyo jacks, and those kinds of moves (I like them in IMAX2, but enough is enough!). I didn't even try the third segment of leg conditioning drills.