Christy Taylor is fun, but I prefer her hi/lo to her step (her step is just too tricky for me.)
Tracey Staehle has a super fun, advanced step workout on the market called "High Intensity Step Mix." I highly recommend it!
Mindy Milrea is a good advanced instructor - she's done some intense interval workouts.
I know others like Seasun Zeiger, but I've never tried hers, so I can't comment.