I haven't played every Assassin's Creed game, but from the ones that I have played, it's between AC4: Black Flag and AC: Odyssey. The former had an exuberant camaraderie between the pirate cast and the beautiful seas of the Caribbean. For Odyssey, I loved Kassandra and the deepest step toward RPG mechanics in an Assassin's Creed game yet.
I am hopeful Valhalla will dive even deeper into the role-playing mechanics, but am just enough excited to explore the Viking perspective of Anglo-Saxon England and the northern lights.