I think Mystery's M3 system is a must have, specially cause it covers most of the necessary fundamentals, also most other guys base a lot of their material on the whole Attraction-Comfort-Seduce system. Which doesn't really BELONG to him but the Mystery Method book covers it well in depth and to my understanding he was one of the first to talk about it. (Don't quote me on that?!)
Style & Gambler have very good stuff. Style was basically my guide to playful routines and multithreading techniques. Gambler has the stealth attraction program which makes total sense out of Kino and physical play.
Also these days I'm watching/listening to a bunch of Sinn's material. If you're an aggressive player and are down to get out of your comfort zone, definitely check out this video. It's about 2 hours I believe.
http://www.youtube.com/watch?v=sbB8AuTBouY
I'd say there's no one system/instructor that works for everyone so keep an open mind and explore different systems while staying true to the basics. I'm sure there's plenty of great material out there that I don't know about but the above has been a good friend to me!
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Guy With 2 Hands -
http://www.youtube.com/user/guywith2hands/videos
"We miss 100% of shots we don't take" - Style quoting someone else!