I haven't played all of them, but of the ones that I have played...
Black Flag- On paper, Black Flag doesn't sound like an Assassin's Creed game. And
technically, it's not great as an Assassin's Creed game. There's less of a focus on the actual assassinations and more on the exploration and naval combat, but it turned out to be the one I enjoyed the most. Edward was a refreshingly different protagonist for the series and I'm glad Ubisoft deviated from the "AC formula" of the past few games, and it proved to be a game that works well on its own but also holds a unique place in the series.
I know, I know. Don't kill me, GAF. I'm
that guy that really enjoyed it. The game has problems but I can't deny that I had a ton of fun with it, and I found myself legitimately interested in the story. I am a huge American history guy so I immediately fell in love with the setting. Pacing issues, a bad protagonist, and a disappointing ending held it back from being my favorite, but I still really enjoyed the game.
Assassin's Creed II- Didn't love it as much as others did, but I played Revelations, III, and Black Flag before this so it didn't have the same impact on me since I was used to the "formula" by that point. That said, Ezio is a great protagonist and I found the setting to be very enjoyable. Considering I played it almost 4 years later, it aged pretty well, and I can definitely understand why it's many peoples' favorite in the series.
Assassin's Creed - I really do like the original Assassin's Creed, and it had a lot of ambitious ideas and concepts. However, the game definitely shows its age and it's just a bit of a mess mechanically. A lot of potential that wouldn't really be fulfilled until later on, but the original game set up a great concept.
Assassin's Creed Revelations- The one game in the series I tend to forget about, but it's still a pretty decent game. It was more of the same formula we had seen for three games at that point and it didn't introduce anything big, but it still brought a fitting end to the Ezio arc. Not a memorable game, but I still enjoyed it for what it was.
I can't really speak for Unity because I only played around 3 hours before growing tired of it. I guess that speaks for itself. Haven't played Brotherhood or Rogue yet, but both are in my seemingly endless Steam backlog and I'm sure I'll get around to them at some point, Brotherhood in particular. I don't plan on picking up Syndicate anytime soon but possibly later down the road when I can get it cheap.