Metroid Prime as an overall game, but a tie with RE4 and HL2 in terms of impact.

God of War 2 obviously.....

Baldur's Gate II - sure, it's not as good as pen&paper D&D experience, but, beside Planescape: Torment (1999), probably closest a video game ever got to it...so far, at least.

GOW3

Only choise for number one is resident evil 4, its a perfect game for that age, pushed gaming foward, changed the industry.

gta vice city, uncharted 2, half life 2, oblivion , mgs2 and 3, ff10, omg so many great games

Metal Gear Solid 2. There are some aspect in it that I've grown to not like as much as I used to, but I think it's still an excellent game, and most importantly, it's more relevant than even with its themes regarding society.

The decade had other excellent games as well. Final Fantasy X is another one that always seem to rival Metal Gear Solid 2, but other titles dear to me include Wolfenstein: Enemy Territory and Titan Quest.

That's a 2010 game, so it's not part of the 00s.