It's hard to name one AC as a favorite, as I personally grew up playing all of them, but I have to say Assassins Creed II, watching the huge gameplay leap from the first one, and Ezio grow up, go through all the stages of loss and grief over his family's death, and grow up and shape to be one of the greatest Assassin's is always something that is going to stick with me.
Also shoutout to the mini-movie Assassins Creed Embers for anyone who wants to watch the conclusion of Ezio's story to the day he dies.