Southern Tier. They're like the supermarket of beers, and I like them all. I've had better versions of some of their beers from other breweries, but they consistently impress me, they have amazing variation, and I can actually get hold of their beers.
Edit: A close runner up would be Avery. I've been drinking a lot of their beers recently and I'm amazed. It seems like they have good variation as well. I wish I had access to some of their less-distributed (less popular?) beers that I see listed.