For me:

Zelda WW and TP
Mario Galaxy
Resident Evil 4
Metroid Prime (1)
Halo (1)
Gears of War

For the entire decade... dang thats a large portion of my life. For 10 years, I will put 10 games.

Lets put on that brain cap...hmmm dang, top 10 is still tough.

1. Ragnarok Online

2. Planetside

3. X-Men Legends 2: Rise of Apocolypse

4. Super Smash Bros. Brawl

5. Sonic Adventure DX Director's Cut

6. Supreme Commander

7. Mario Kart Wii

8. SSX Tricky

9. Rock Band

10. Wii Sports

Well if I was purely speaking on trying to figure it out for the entire industry, I'd pick Half Life 2 in a heartbeat. Game was just incredible. But on a personal note, mine was quite a hard choice but I'd probably go with Resident Evil 4 with Half-Life 2, Twilight Princess, and Galaxy right behind it.

Oh and let's just end this argument now. A decade is defined by the ten digit. Meaning if they are all the same that qualifies a decade. 00, 01, 02, 03, 04, 05, 06, 07, 08, and 09 defines the last decade. This isn't debatable haha. That is how you define a decade and why we use it. It only makes perfect sense... let's not argue with logic.

Persona 4.

Oh i forgot

Age of Empire II/Conquers and Age of Mythology

...:DEMON'S SOULS:...
[follow ups: Zelda WW & Mario Sunshine]

Good pick, I also forgot about this. Awesome game.

Not true.

A decade is defined as a span of time of ten consecutive years, look the word up in any dictionary. The bolded is just the most common meaning when further qualifiers are not added to an expression like "the 60's decade". Also, logic has nothing to do with definitions per se, only with how the terms you define are manipulated in relation to one another. As such, a definition can be less useful than another for very down-to-earth reasons (in this case most people means 0-9 when speaking of a decade and the similitude with centuries adds little worth in any common discussion) but not illogic.

OT:

My game of the decade: SotC (HL2, Diablo II, Starcraft)

Game that really shaped the decade in gaming: WoW (GTA, The Sims, WiiSports)

For me, it's Gran Turismo 4 or Uncharted 2. I just love them both so much.