Seasun Zieger is great for complex choreography. Straight Up Step would be the most moderate of her choreography. My favorite of hers is CIA 2903 - Stepping Up. It has great music. This is probably my most complex step workout and it takes awhile to learn the choreography (you probably won't get it on your first time out unless you're a real master stepper!) but it really is worth the effort.
I also like Tracey Staehle's Steady State Step.
Rob Glick is also good. I like his Amazing Step Styles.
And last but not least, Patrick Goudeau. If you can handle all his "Oh my goshes", he is so much fun. I like all his step workouts except for B.E.S.T. which I thought was really hard to follow and I really love complex choreography.
I would rate all of these workouts as more complex in terms of choreography than Cathe. I find Rhythmic Step to be her most complex and all of these, IMO, are more complex.
Just a warning - Patrick and Seasun love to turn! But you could always leave out the turns if you don't like them.
I also agree with Cathe about the Evolution instructors. I like Robert Steinbacher's Step Sensation. There is an Evolution DVD called Stomp It! Step Showdown 2 which has the following instructors and excerpts from the following workouts:
Rebecca Small - Symmetrical Step Solutions
Yoav Avidar - Contagious Choregraphy
Adi Rothman - Contagious Choreography
Bianca Swartout - People in Motion
Marcus Irwin - Dance Fusion
Mark Oliver Kluike (Really like his stuff!) - Dance Step 3
Petra Kluike - People in Motion
Jutta Schuhn - Burn & Tone
Steve Schiemer - Combo Mania
Marcy