Sarah is one of my favorite. I love Candlelight Yoga, Fatburning Yoga and Joy of Yoga.
Eoin Finn is another good one, but more advanced.
Yoga, Mind, & Body with Ali McGraw (I believe the instructor is Erich Schiffman) is also good.
I'm a yoga newbie, so haven't tried a lot of instructors, but enjoyed these the most.
Andrea