top 10, reordered by year

2000, 5, Deus Ex

2003, 12, EVE Online

2004, 3, Half-Life 2
2004, 15, World of Warcraft

2006, 17, Hitman: Blood Money
2006, 19, Company of Heroes

2007, 4, Team Fortress 2
2007, 7, Bioshock
2007, 14, Portal

2009, 9, Left 4 Dead 2
2009, 13, ARMA 2

2010, 2, Mass Effect 2
2010, 10, Starcraft II
2010, 20, Civilization V

2011, 1, The Elder Scrolls V: Skyrim
2011, 8, Minecraft (first public release 2009)
2011, 11, Deus Ex: Human Revolution

2012, 6, XCOM: Enemy Unknown
2012, 18, FTL

2013, 16, Bioshock Infinite