My personal favorite is Clinton Anderson. I have to say though, if you're looking for a "no gimmick, no nonsense" approach to training that actually works
, go with Chris Cox. Clinton is great but heavily relies on you buying the halter, stick, string, $600 dvd set, and so on. Chris Cox just uses his rope halter and lead rope. He doesn't try to sell you on his program the whole time either, he's just training the horse (and quite well, I might add). I've seen him a few times in the past couple years at Equine Affaire and I was really blown away by him.