One of the ENnies judges described A Song of Ice and Fire as the best product he read during the awards evaluation. Given that there's close to 300 products submitted, that's high praise.
Maybe not the best -- too many products and too much variety for me to pick the best -- but probably my favorite. The one that most made me go 'ooh, I want to play with this'. A really nice blend of the tactical and crunchy with still some heroic and narrative bits thrown in. Unified mechanics for personal combat, intrigue, and mass combat. A character generation system that accomodates the old knight and the young squire in the same group, and still makes them both effective in different ways.
Honestly, not a big fan of the setting. I've grown rather weary of the novels, and Westeros is not especially conducive to gaming in some regards, IMO. But it was well represented in the book, and very smart of Green Ronin (read that out loud for me, Piratecat!) to base it when they did before the first book. It gets the flavor and the history without overly hamstringing the GM or the players.
I'm working on adapting the rules to an old, old homebrew (some have seen bits of it in the Winter Witch one-shot). I'm thinking of having each player make a house in a recently conquered area, and then create 3 or so characters for their house. There would be external threats and so forth to make the different houses want to cooperate on some things, but still be a little competitive.
Fate point spent by characters would be tagged with the House, and then go into a pool. When they refreshed, I'd redistribute them randomly, so you may end up with a Fate point tagged with another House. The player could then use that during the 'House Actions' parts to either boost their own House's fortunes, or surreptitiously screw with another House.