Probably Black Flag, although I must admit I haven't played 1-3.
Unity was definitely right up there with Black Flag for me in enjoyment. Maybe because I was still new to the series, but I definitely felt that Unity was a strong game (despite all the criticism).
Syndicate was pretty weak imo.
Haven't gotten around to Origins yet, if it got ported to the Switch I'd hop on that so fast though! May get it on PC when I need a game to play.