I'm gonna have to say my top 5 cuz there is no way anybody can pick the top game of a decade (too many games and genres to do so).
so here it is:
What appears to be just another run-of-the-mill FPS turned out to be what revolutionized the genre. While the game mechanics, story, and many of the weapons remained the same, what made the game great was it's ability to unite gamers together through online matches on consoles and make make death matches more popular. So while gamers may question if the game is any good or not, what cannot be questioned is it's importance to gaming history in the past decade.
4) Zelda: Wind Waker
After E3 showed an adult version of Link battling Ganon on Gamecube graphics, people went nuts. When the game given to the gamers showed the little kid with cel-shaded graphics, people were ready to grab their pitchforks and torches. But when people calmed down and played the game, it turned out that the same Zelda magic was still there, just with a different mask. A wonderful Zelda game, the game proved that you didn't need graphics of realistic proportions to have a great game and provided a new technique in graphic designing to prove it.
3) Resident Evil 4
Much like Wind Waker (only through a different venue), Resident Evil 4 provided new legs to a dying franchise. Let's face it; the first couple of games were great but the formula was getting old fast. So what did they do? Changed the game from a horror design to more of an Action-Adventure genre with zombies and they haven't looked back since. And why should they?
2) Star Wars: Knights of the Old Republic
One of the greatest Western RPG's of all time, and it was a Star Wars game? Yup and it was great indeed. Being able to choose to go either the Light side or the Dark side, and a compelling gameplay mechanic with choices and a twist or two in the plot, and you got the a great game!
1) Grand Theft Auto III