Allagash, easily. They're incredibly innovative in so many ways, from the only coolship in North America (and one of the pioneers of NA spontaneous fermentation) to their willingness to play with wits and not just clone Hoegaarden, to the whimsical Four and Interlude.
In fact, between Allagash and what Geary's has done exploring UK styles, (not to mention Shipyard and Stone Coast and Casco and the other breweries doing yeoman's work), and the Great Lost Bear's 60+ taps (and 3-4 cask-conditioned real ales) there's an amazing amount of top-notch brewing and beer appreciation going on in the area.
In fact, I'd say that Portland, ME is the most impressive city named "Portland" for beer and maybe my pick for #1 beer city in the country. Given the impressive work being done in Portland, OR, that's saying something.
Also doing amazing brewing work: Stone, Flying Dog, Schlafly. Honorable mention to the woefully underrated Weeping Radish. verrated award goes to the quite good but not great Dogfish Head, though they deserve high marks for inventiveness in their specialty/seasonal/one-time brews.