World of Warcraft probably wins it for the impact, I like Doom 3 but not enough for GotY, I've enjoyed Rome: Total War a notable bit, Metal Gear Solid 3 is a great game in a series I like a lot, but ultimately Half-Life 2 gets my vote. Of all the candidates, it has stood the test of time the best in my opinion (although graphically Doom 3 still beats it by a fair margin).