If you're not averse to getting messy, Summer Shack is the place to go. Big family-style atmosphere with great fresh fish and reasonable prices. It's not a stuffy high-end restaurant, but maintains very high culinary standards.
Locke Ober's Lobster Savannah is world famous. How do you make a delicious (and massive) lobster even better? Stuff it! The stuffing is--by itself--to die for. If you want to show an out-of-towner lobster, then Locke Ober is the place to go
Obviously the question is about Lobster, and McCormick and Shmick's certainly has the best. They print their menu's twice daily with whatever is fresh that day. Truly one of the best seafood spots in Boston (yes, I know it's a chain, but I am always impressed)
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