Hmmmmm....
-The IMAX 2/Cardio and Weights DVD is a favorite
-IMAX 3 is tough and a favorite
-IMAX 1 is tough, (but the fun factor is not there for me) and comes with Rythmic Step and MIC (I have these on video not DVD),both good workouts.Rythmic Step is more complex and dancy and MIC has a killer hi/lo segment followed by a pretty tough step routine. I enjoy these workouts, but they are not favorites.
-KPC/Legs and glutes is a favorite and has some really awesome premixes blending the two workouts into circuits
-Step Blast and SJP are also a lot of fun, but I don't think they are as tough as the IMAXs. Step Blast is more dancy and complex and has a blast section at the end. SJP step is not very tough, but it is fun and the hi/lo is not as killer as MIC's, but the music is better IMHO. Some really good premixes on this DVD also.
-Cardio Kicks looks fun, but I have not done it yet, just previewed.
-I also really enjoy the CTX cardios. They are short, but tough and can be combined into longer workouts. There is a variety of step, hi/lo and kick boxing--very versitile.
-Circuit Max has some really tough cardio also, but it is broken up by weight segments and not well chaptered, so if you wanted the cardio only, you would have to fast forward through the weights. Although Aquajock's suggestion is to do heavy leg work during the weight circuits instead of the light upper body work that Cathe does.
She has more stuff also. I think on the video/DVD comments board (right below the ask Cathe board) there is a sticky which lays out all her workouts from "easiest" to hardest. You might want to check that out for more information.
You also might want to post this question on the general forum. You do a lot of running and you might want the opinion of other runners on their favorite Cathe cardios.
Maggie