There's been many and many have been less significant that others, and yet, in many ways as important. For instance, I can't say that Swedish Mikael Frödin has had a direct impavt on me (although I've met him several times and he's a very nice man and a gentleman), but still - most of my salmon flies are tied using his principles. Like that Mikael Frödin has had a gigantic impact on most Scandinavian fly fishers.
Another is Oliver Edwards, whom I still consider one of the most imortant innovators of trout flies in the modern era. I've met Oliver on several occasions having attended several tying courses here in Demark with him. His book was and still is, also something of a milestone. Still a work of reference for me. And let's not even get into his movies, which are still absolutely amazing!
Early in the 2000's, I worked in a Danish tackle store, and one thing wee did was that we brought in a lot of American books - flytying books, mainly. A book that really "made an impact" was Shane Stalcup's "Mayflies - Top to Bottom". The book features Stalcup's innovative tying techniques, all relatively simple. many featuring some signature materials, most noticeably Medallion Sheeting. I really enjoyed that book, and still do.
I still tie and fish many of his patterns.
All this is leading up to this. Just recently I found a channel on YouTube that hosts many tying videos with Stalcup. If you haven't seen his flies, check out the videos, and if you already know the flies, it's nice to see Stalcup tie them himself.
Shane Stalcup passed away prematurely in 2005, aged 49.
Have a nice weekend!