I love Eoin Finn's Yoga for Happiness - it's one 80 minute workout that has several premixes (and you cannot tell they are premixes at all). He does a magically hips section that opens my hips and has increased my flexibility better than anything. It's very doable - fun and light hearted, but challenging. He gives many options for poses, as well. He does say some weird things, but I just shrug them off.
I also like Barone Baptiste - he's a little chatty, but very sweet and knowledgable. I have his Long and Lean which while not a Power Yoga DVD is still a good, tough, workout.
A lot of people like Bryan Kest but he drives me batty.
I use Rodney Yee's Power Yoga for Flexibility a lot as well - it's short and a great after run or hard workout stretch.
I think a lot of it depends on what you want to accomplish. I lean towards instructors who really stretch me out, and work on my flexibility. I am not a fan of very intense moves like the headstands, etc. I'm also not a fan of yoga that turns me into a pretzel. But, I want a good workout and I want to enjoy it. Others like different things.