Fallout 3 at this point. Not only is it a really excellent game, it's packing tons of value with all the different DLC available. And let's not forget the indescribable pleasure of having Danny Kaye songs endlessly stuck in your head.
Now that the game's been slashed to $24.99 for the weekend on Steam, you have less than no excuse. It's like ... negative excuse, which we believe means you're legally obligated to buy a copy, even if you already own one.