Depends on what kind of 'experience" you''re looking for....
Boothbay will give you access to a "walkable" downtown village area with multiple shops, restaurants, etc. and plenty of chance to look at the various boats in a good-sized harbor. Also access to one or more of a variety of cruises of various lengths.
Sebasco is basically a self-contained resort, several miles for anything anyone would call a "village" or "downtown" or any sort. It's a nice resort, but it's geographically "off by itself" -- if you're looking for a "quieter" experience and can be satisfied with the features and activities at the resort, it may be just what you're looking for. But if you want to "stroll around" to see what else is around, there's not much"else" around.
Harpswell is a rather spread-out coastal community -- much more rural than Boothbay. Again, if you want peace-and-quiet and can settle for what's immediately around you, it may be fine. But many of its "features" will require a car ride to get to (from one part of town to another).
Hope this helps you understand that the three options you mention are really three different experiences. Which is "best" depends on what you're looking for and what your expectations are.
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