Left 4 Dead.
Valve really surprised me with this game, the AI Director is amazing. You can play through the campaign 40 times, and every single time it will be different. The graphics are insane, the blood and gore is ramped up to 100, the creepyness is a little low, except for when you hear a witch the first time you will have the **** scared out of you. The witch is by far the most creepy thing I've seen in Left 4 Dead. I loved the tanks, boomers, and smokers' attacks in versus. The most amazing thing in the game though, is the replay ability. You can play through every campaign, and still want to play it over and over again. I also loved the fact that if you're in a tight spot, just take out a molotov or pipebomb, and you can clear out the room. I have saved me and my friends' asses so many times by using them for tight spots. The multiplayer in Left 4 Dead is incredibly fun, and if you like to make people really mad, you'll love this mode. You can play as a Boomer, Tank, Smoker, and Hunter. Hunters pounce on their victims, Hunters throw their victims yards away into a solid wall, a boomer vomits on their victims, which alerts the horde (normal zombies), and a smoker graps you with their incredibly long tongue, and will not let their victims go unless if they are killed. My personal favorite is the boomer, just to see tons of zombies rushing for your enemy. It's a great feeling just to **** up people's good time.

Left 4 Dead deserves the "Best Game of 2008" Award. Hands down, it is the best game of 2008.