I've been fortunate to do workshops with a few people, including Boon, Roy Nagatoshi, Ryan Neil, Steve Tolley, and Walter Pall (through the Natures Way events):
-- Boon - I took a repotting workshop with him. He was a really fantastic teacher, and I learned a ton from him. He even was nice enough to stay a little longer and work on my juniper a bit with me, and teach me a few things on juniper care and styling. It's really hard to find a nicer guy.
-- Roy Nagatoshi - I thought this was a big waste of my time and money. He seemed more interested in chit chatting with others and working on his friend's trees rather than spreading his time and helping out the rest of us. If anything, I would only try to see him for grafting work.
-- Ryan Neil - He was absolutely fantastic. He is also a really great teacher, so good in fact that you would think that he had been doing it for decades. He really made you think about the "why" something was done or should be done, which I thought was a great approach.
-- Steve Tolley - I took a workshop with him where we worked on recently collected yews a few years ago. Very nice guy, but I probably didn't get much out of the workshop as, in fairness to him, I'm not a big fan of a workshop where you're given brand new material, and feel like you have to style it in a few hours (let alone, even pick a front and a style). It was educational to watch him do carving work using a die grinder on another's tree.
-- Walter Pall - I went to one of the Natures Way events, and had a really good time and learned a lot. He really didn't do any of the work, but sat back and let us work on some seriously expensive trees under his guidance - it was a priceless hands-on experience.
For my wish list, I'd have to also say Graham Potter and Hans