Powermax is my favorite step workout and Cathe breaks it down more, so is easier to learn. It's on the Cardio Hits DVD. Step Fit and Step Works are fun too. Step Works is a faster tempo, and would probably be the last one to conquer on the DVD.
For circuit, I would suggest Cardio and Weights. The step segments are fun, fun, fun! And since they are short, it seems more learn-able and do-able. Good upper body work on this one too. IMAX2 is on the same DVD and is an interval workout that is motivating because of the music. Of the others that A-Jock listed, the easier ones would be Step, Jump, and Pump (on same DVD as Step Blast) and High Step Challenge.
Diane