Half Life 3!
Totally game of the year, played from start to finish, took 2 days and I don't think work bought my "sudden flu", but it was totally worth it. I know it's technically only 'early access' right now and they haven't technically released it, but it's open to anyone and I didn't find and bugs or clipping or glitches or anything. Funny that they've snuck it out without any fanfare whatsoever (which is completely insane, but then it is Valve) so it hasn't had as much time to sink in as Bioshock or GTA V, but it was just ace. Worth the hype? Maybe, maybe not, but it blows Bioshock and Grand Theft Auto out of the water.
Surprised there hasn't been any rave threads or reviews on it yet because it's been up for a few days now, I'd expect it to have gone viral already, but maybe you lot just don't surf Steam as much as I do:
http://store.steampowered.com/app/244210/ [https://www.youtube.com/watch?v=SbYXkOAoZpI]