3 is very similar to Black Flag in terms of gameplay. though the naval missions are contained, linear levels rather than the main focus of the game. Combat is similarly largely about timing counters for quick kills rather than the Level-based action combat of Origins onwards, though still fun if slightly repetitive.
Origins for my money is a must-play if you enjoyed Odyssey and Valhalla. The
RPG elements aren't there in full yet, but the combat is wonderfully brutal, the writing is really solid (an advantage of the linear nature) and the two lead characters are some of
AC's best in terms of both the actor's performances and their stories. Also the game world is one of
AC's best, vast and open and just stunning to look at.
Syndicate, by contrast, is the last hurrah for the older style of
AC game, and really, it's just quite fun. I never finished it and found the plot rather bobbins (which, to be fair, is par for the course for
AC), but the leads are fun, the combat is a little better developed and hey, you get to ride round London on a steam train, so that's always a plus.
I can't speak for Unity or anything before 3, though.