I also have all of Cathe's cardio but I also have a lot of non-Cathe step, which I use for variety. I agree with those that recommend Amy Bento's step. Of her three step DVDs, I'd recommend ASC2 and ASC3; the music and choreography are good and they are fairly intense. I don't like the original ASC because of the camerawork (the angles and isolated body-part shots make it very hard to follow!)
My other current favorites are:
Seasun Zeiger -- Rock Steady Step
Deirdre Morris -- Tap Less, Step More
Sharon Twombly -- Athletic Step Jam
Pam Cosmi -- Steppin' In Rhythm
Rob Glick -- Amazing Step Styles
As for Christi Taylor, I have most of her step DVDs. I enjoy them, and I think they are worthy additions to any step library. I'd rate her as high intermediate, rather than advanced. However, Christi does have a couple of major drawbacks for me (although these don't seem to bother some): 1) she teaches new moves by saying "Watch me" (I cringe every time I hear this!!) and 2) she does a LOT of TIFT'ing (taking it from the top). For those reasons, I don't use her workouts very much.
Hope that helps!