Well, it used to be GTA: San Andreas.
But now it's GTA 4.

Mostly because you can pretty much drive off and do any random shit you want, and it isn't limited by a lower age classification
My personal favourite thing to do at the moment is trying to bomb people by launching a flaming motorbike at them from a ramp. Very, very hard.
Also + for ragdoll physics. It's awesome watching people tumble after getting hit by a car or flying bits of scenery.

Well, either GTA 4 or Just Cause 2. Just Cause 2 mostly for the same reasons as GTA 4, with bonus grappling hook function.

I love me my sandbox-free-roaming-do-random-crap games.

Timesplitters: Future Perfect and Dynasty Warriors 5

Toribash. Loljk. Warcraft 3 is pretty cool. Modern warfare 2 too ^_^

Mine is Empire Total War and Rise of Nations

This message has been deleted

My new one is Red Eclipse.

Awesome game, made a thread about it, y'all should check it.

Lets see:

Borderlands, Gears of War 1 2 and soon 3, Team Fortress 2, All Half-Lifes, Skate 2 and 3, Bulletstorm, Portal, Splinter Cell: Conviction, Assassin's Creed 2 and Brotherhood, All Legend of Zelda games, All super smash brothers games, All StarFox games, Starcraft 1 Brood Wars and Starcraft 2, Minecraft, Killing Floor, Just Cause 2, All Pokemon games (Except Ranger), CounterStrike 1.6, Crysis,

LIST GOES ON. There is no way for me to pick my absolute favorite.

Just this.

As for me, I would say COD4.
I just go back to it so many times. That and it's one of the only FPS's now that people still play, which also has LAN, game developers have completely forgotten that some people have crap internet and the only way for them to play with other people is with lan.

Call of Duty MW2

Mass effect 2
without a doubt