Assassin's Creed 2 as of right now for sure. I began with the original game, and while I still love it and it holds a special place in my heart, 2 was just the perfect sequel. Following Ezio from his birth, to his arrogant teen years, to finally becoming a wise adult and learning the values of the creed, it was beautiful. Incredible soundtrack by Jesper Kyd, who is also returning for Valhalla, improved mission design from the first, awesome parkour and exploration, it's one of my favorite games of all time period. I'm looking forward to trying Valhalla because it seems like it's finally a slight return to form after two RPG games.