Valve definitely need to be considered for the title. It's not flawless, but 15 years later and they are still hitting the same quality ^^ Most notable:
1998: Half-Life
1999: Team Fortress
2000: Counter-Strike
2003: Day of Defeat
2004: Half-Life 2, Half-Life 2: Deathmatch
2006: Half-Life 2: Episode One
2007: Half-Life 2: Episode Two, Portal, TF2 (dat sweet sweet 2007 @_@)
2008: Left 4 Dead
2009: Left 4 Dead 2
2011: Portal 2
2012: Counter-Strike: Global Offensive
2013: Dota 2
Edit: Thinking about it, Naughty Dog definatly need to be considerd too, they seem incapable of making bad games.
1996: Crach Bandicoot
1997: Crach Bandicoot 2
1998: Crach Bandicoot: Warped
1999: Crash team racing
2001: Jak and Daxter
2003: Jak and Daxter II
2004: Jak 3
2005: Jak X: Combat Racing
2006: Daxter
2007: Unccharted
2009: Uncharted 2
2011: Uncharted 3, Uncharted: Golden Abyss
2012: Jak and Daxter Collection
2013: The Last of Us
