I don't know as I can really cast a vote. I have learned so much from so many different sources. This year is a good example: I got to ride and work with some really great farriers, learned a lot from each of them. I attended the Summit, got some really good learning there as well, both in formal and informal discussions. I take the American Farriers Journal and the The Farrier's Journal over here. Very good articles in both. As stated just getting together with a few other guys, pounding iron, talking and such is great. In preparing for my Guild Certification and the Danish Certification I learned a whole lot and improved my skills as well.
This forum is also a great learning place. I guess I'd have to check most of them.