AC2 is probably the best AC game. It made tons of improvements over the first game and definitely has the strongest story in the entire series. Plus it's the start of the ezio trilogy. Brotherhood improved combat a lot but has a more average story. Revelations is irrelevant except for the first and last cutscenes of the game. Improved multiplayer a lot though. 3 was different. I liked it but most people didn't. Black Flag was super but didn't have a lot of focus on actual assassinations. Mostly known because the ship combat was so fun. Never played Rogue. Unity gets a bad rep because it was broken at launch. But it's fixed now obviously so most peoples' dislike for it is no longer valid. It brought back actual assassination missions that are really fun. I might choose that after the ezio games. Syndicate was fun. Introduces new movement options which really make the game more fun. But the story is still kind of weak. So I'd go AC2 myself. One of my favorite games ever, let alone in the series.