Assassin's Creed IV: Black Flag, for my money, is the best in the series. It's a good Assassin's Creed game, but importantly, it is also a brilliant pirate one. Its world is lovely and big and has side quests and objectives that are absolutely worth doing. If you've not played it yet, it's definitely something you should sink your teeth into. You have no excuse after next week, either. Ubisoft has announced that Black Flag will be available for free on PC on uPlay for its seasonal Playdays event.
All you need to do is sign into the service sometime between December 11th and December 18th and claim your copy. If money is a little tight at the moment but you want to get something to play over the holidays, I highly recommend it. It's ace.