Not being a really avid gamer, I'm not sure what title I'd put ahead of it a "Game of the Decade". I've never played Ico or Shadow of the Colossus, for example, so I can't say that either of those would be suitable candidates (I'm not saying that they CAN'T be, only that I can't judge either way).
So, I guess, what I'm asking here is, "if you don't think Mass Effect 2 deserves to be the Game of the Decade, what game would YOU like to see there, and why?
I'm actually torn between Halo, Gears of War, ME2, Dragon Age: Origins, Half Life 2, Left 4 Dead, and Portal of the games that I've played personally, and Knights of the Old Republic and World of Warcraft thrown in for good measure.
As for WHY:
Halo: The gameplay style set the standard for shooters, with limited weapon choice, partially regenerating health, etc. Many of its innovations are still being used today. Not that this is necessarily a GOOD thing, but still...
Gears of War: This game effectively revolutionized console shooting games for the 360/PS3 generation. Other games prior to it had cover-based shooting and regenerating health, but Gears refined the mechanics and made them a new standard (for better or for worse). Like Halo: Combat Evolved, it was a HUGE influence on subsequent games. And, like Halo, whether this was a good thing or not is subject to debate.
Mass Effect 2: A very strong mix of combat, characterization and story. Plus, the single best use of cutscenes I've ever personally experienced in a game. During the final mission, the cutscene interruptions actually enhanced the entire experience, where cutscenes interrupting my attempts to shoot/stab/curb stomp somebody usually shatters the immersion and annoys me.
Dragon Age: Origins: I loved the characterizations, I loved that you could play the same type of character several different ways, I loved the fact that, by the conclusion of the game, telling somebody how it went down sounded like a summary of an epic fantasy story. And I loved the dog and Shale.
World of Warcraft: A game I've never played, and have zero interest in playing. Nonetheless, it is one of the most popular, well-known, and ground-breaking games in existence and, to date, appears to be the pinnacle of MMO design.
Knights of the Old Republic: Never played it, but I know it by reputation, often by gamers comparing Mass Effect unfavorably to it.
Half Life 2: One of the most immersive games I've played. I felt exhilaration (and vengeful glee) when shooting down that helicopter that had hounded me throughout the canal system. I felt paranoid terror while being stalked in Ravenholm. I felt like Fathor Grigori deserved a memorial as he sent me to safety while he laughed and turned to tend to his flock one last time while the headcrab zombies overwhelmed him...
Left 4 Dead: Typical Valve game: Incredible sense of immersion, good level design, and polished to a mirror's shine. I STILL don't like playing it after dark.
Portal: Because it's PORTAL, and it would take a wall of text to do justice to its greatness.
Now excuse me while I go find a crowbar and pry my lips off of Valve's butt...