Having just a ton of fun with Assassin's Creed II. There's always something new and interesting to do. The story missions have been a great source of entertainment, and the story that takes you from city to flooded city is full of action, romance, mystery, and more than a few laughs. I've made it maybe halfway through, and the mystery is really heating up, providing more backstory into Desmond's ancestry and the ancient conflict between Templar and Assassin.
However, probably the most fun I've had is filling up the family estates with an incredible amount of collectibles. The Villa Auditore at Monteriggioni holds all the paintings, codex pages, feathers, seals, weapons, and armor that you collect throughout the world. They're all rewarding, especially the art gallery, where you can browse through the Renaissance artwork you purchase and collect in your dealings. The status of the estates provide a valuable mark against you can gauge your progress within the game, and gives a great deal of satisfaction when you can cross something off your list. It's one of my favorite parts of a nearly flawless game.