Scott in Falcon
I almost feel under qualified to answer this question as there are so many demonstrators that I haven't seen yet but would like to. Anyhow, based on those that I have seen, here are my picks:

Jimmy Clewes - By far, my favorite demonstrator. He is a story teller and an instructor at the same time.

Stewart Mortimer - He makes everything look easy. He makes you think that you too can be whipping out spiral pigtail finials in 2 minutes or less. It isn't until you are at home trying to do what he did, that you realize just how difficult it really is and just how good he really is.

And, I've got about 5 or 6 people that I would like to list for number 3... but, if I have to pick one...

Trent Bosch - Trent gets excited about demonstrating and his excitement is contagious. I always leave his demos ready to go get a cool new tool and try something that I never even thought of before.

Looking over my list, all three have something in common. In addition to a mastery of their skills, every one of them engages with the audience really well. Even though you are sitting with a group of people, you feel like they may as well be there just talking to you!