Wow tough decision. I own RS, MIC, and Imax on VHS but want them on DVD. I think they have some of the best killer combo's and it would be fun to mix and match them. If you want just tough classic Cathe cardio go with RS/MIS/Imax and I think the original Imax is the toughest of the three in my humble opnion.
I agree with Shirl though, if I HAD to pick right now I'd probably go with SJP/SB because in addition to two great workouts, you get a lot of fabulous premixes. One of my favorite premixes is the cardio one where you alternate between SB step and the hi/lo sections form SJP. I can't remember the name right now but it's right at an hour and just a perfect little cardio premix.
Imax 2/C&W is pretty awesome though. Imax 2 is a lot of people's favorite Imax workout and it's tough but doable tough with really fun music. C&W is not too tough at all, but more of a good intermediate circuit workout. Some really nice upper body work and I do like the core section of C&W.
If I had to pick right now this would be my list from favorite to least (with least favorite being used LIGHTLY as I enjoy all the workouts you listed)
1. SJP/SB
2. Imax 2/C&W
3. RS/Imax/MIC
Good luck with your decision! I'm sure you'll be happy with whatever you decide to purchase.