Half Life series for me. It might be hard to see from this time, looking back, but Half Life more or less created the modern shooter. It was one of the first games to have complex AI that reacted to what the player was doing. Black Ops soldiers pitching grenades at you when you hid from them, falling back when they were wounded, and trying to flank you was TERRIFYING at the time. Gamers had never seen anything like it. There aren't many modern games that I love playing through again. Half Life and Half Life 2 especially... I just did a run through HL2 and it's episodes over the weekend. And it is still as amazing as it was the first time. It's a real shame to say it, but HL2 is still THE bar in a number of ways. While other games have surpassed it in graphical fidelity, few if any at all have provided a story and cast of characters that are anywhere near as wonderfully human as HL2's. The game then ingratiated itself to the public further by having a fairly open, standard, and easy to use set of mod tools and an aesthetic that lent itself to all manner of new games. Thanks to Half-Life we got the likes of Team Fortress Classic, Counter-Strike, Cry of Fear, and countless others. By the early 2000's, Half-Life wasn't just a game but a functional platform. I'm looking forward to Halo 4, Dark Souls(pc), Dishonored, Darksiders 2, Sleeping dogs, Watch dogs, Bioshock infinite and Borderlands 2.