this is the narrowest:

Resident Evil 4 (Wii)
Gears of War
Far Cry
Metroid Prime 1
Elder Scrolls 4 : Oblivion
The Legend of Zelda: Majora's Mask

The Legend Of Zelda: Majora's Mask

wow that's a tough question... and the answer was the first word... Just because that game took so much of my time... more even than CS.
Any game where you can count your gameplay time in months is bad for you...


(and thanks for reminding me that I never continued my play of masquerade... it was indeed great and deserving of me going back to it).

Demon's Souls

Shadow of the Colossus

Chrono Trigger DS and Twilight Princess

If 2010 is considered part of the decade ( As some do ) then it's not finished.

But if not then Forza Motorsport 3.

Followed by Mass Effect 1

And 3rdly : Halo 1

Hmm... thinking on this for a bit, I have to say Persona 3:FES. Yes, P4 is a better game, but it was FES that I got to first, and it's what made me a raving Atlus fanboy. It was the game that showed me that yes, there are still games out there that focus on a good game and challenge. I've been going back through more of Atlus' catalog, finding more and more gems. And I'dve not found them if it weren't for FES.

world of warcraft

OMG

I forgot that game released in 2000

*edits post*

Although I believe there is some debate as to when the decade starts....either 2000-2009 or 2001-2010