Topic.
In my opinion there are two other games that are more influential. Halo: CE and Grand Theft Auto III.
GTA 3 literally changed the landscape of gaming. It created the first truly engaging sand box on a console.
I like Halo more, but GTA 3 was the most important game of the last decade.
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It really was. The Half Life series(And any game by Valve, frankly) has revolutionized first person shooting games with a number of new concepts. It also helped the Halo series along.