Easy: Assassin's Creed II. I can't really imagine playing anything else until I've completed every assassination contract, looted every treasure chest, explored every tomb, plucked every feather, decoded every glyph and of course avenged the Auditore family.
I like to take my time with open world games, completing as many side missions and optional objectives as I can in between each primary task. Perhaps the depth of my love for Assassin's Creed II is down to the sheer quantity of these secondary activities.
Yet it's not just how much there is to do, what also impresses is the diversity of these challenges and the manner in which they're so cleverly integrated into the world. Sure, the treasure chests are an obvious collectathon, but you buy treasure maps from the town art merchant. Shooting pigeons in Liberty City was tedious, but here you can find assassination contracts in pigeon coops, unlocking some of the game's most freeform gameplay upon accepting the contract.
I've nearly upgraded my villa to its full capacity and have been working my way through the assassinations in Tuscany. Still haven't gone to Venice yet. It's an exceptional game.
What are you playing this weekend?