I don't care for any of the trainers who have their own product lines, but I have to say that Parelli is probably my least favorite of them.
He talks a lot, but doesn't say anything worth listening to.
I still remember the first time I read an article about Parelli "hill therapy." I thought it was a joke. . .like the horsey version of an article in The Onion. He and LP really seemed to think that they'd "discovered" the concept that working a horse on a hill was a good way to build muscle and balance, and they were "selling" the concept to their loyal followers.
Anyone who has done any riding outside of an arena on any kind of a regular basis knows that working over varied terrain is great for both horse and rider. You don't need to join a $20/month "club" to be told that.
As for favorites, I have a lot of respect for Buck Brannaman, though (from what I've read and heard) he really doesn't consider what he does as "natural" horsemanship and regards the phrase as little more than a marketing strategy.
"Parelli horsemanship is just like painting by the numbers. You need absolutely no skill. You just put this color here and this color there, and when you're done, you have ... a mess no one wants." mp