Ac 2 and brotherhood are by far the best imo. 3, black flag, and unity once fixed are also great, origins was the last really good assassin's game.
Odyssey is a great game but it's barely an assassin's creed game and set up a lot of the terrible staples Ubisoft would adapt going forward.
Valhalla is in no way shape or form an assassin's Creed game and just plain old sucks lol