If this thread is limited to New England towns, I'd nominate the following:
Hanover, NH - Sure it's a cliche college town in many ways, but in my own opinion Dartmouth has the most picturesque of all the Ivy League campuses (though I haven't seen Cornell's, which very well might take the cake).
Ogunquit, ME - Beachside resort town, with less distinctive urban architecture than most New England towns (it's TINY), but it's still lively and the housing is attractive.
Concord, MA - A bigger version of Ogunquit: less remarkable for its downtown than for its well preserved history and historic homes. Low density but still well-forested and a distinct if small center.
Connecticut needs some more nominees...