AC II, Brotherhood, and Revelations are compulsory games, in my opinion. Ezio's a fantastic character, and you get to experience his life, literally from his birth to eventual retirement and death. Those three games are the pinnacle of the series, so I'd say, if you need to pick and choose some games, just stick with those.
Black Flag is another good installation with a emphasis on pirating and Naval battles which are immensely fun. There are also some forest sections that remind me of Uncharted 1.
Just be warned the games are VERY buggy, and the AIs are some of the stupidest I've ever seen
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I also have to say, I love the musical scores in AC. The series has had some fantastic composers like Jesper Kyd, Brian Tyler, and Lorne Balfe.