I'm going to say that Grafton is one of the freest towns anywhere. I don't know if their taxes are lower than Seabrook or not, but I could imagine that. Folks from that area have said a lot of good things about the town, and there's a Reasons to Move to Grafton thread (like Seabrook, Portsmouth, Manchester, etc)
My preference, of course, is for small towns with low taxes that are a short drive from beaches and cities. Obviously, that leads me to Seabrook, which should be on anyone's check out list... but Portsmouth should be, too. If you can't afford the rent in Portsmouth, there are quite a few towns in the area (like Seabrook and Hampton) that offer less expensive housing options.