Mass Effect 2 without a doubt.

Dragon Quest IX or
The Last Guardian

Everyone that picked FFXIII, why?

I've maxed each FF I've played and I can't see playing any FF for a year unless it's FFXIV as an MMORPG. But Standard console FF for a year, man that's harsh.

IMHO I can't see any non MMO type being played for 1 year and nothing else.

Global Agenda

From April 2008 through to May 2009 I basically played Mario Kart Wii with a few strategic breaks for some other games.... I would not have been that bothered had I not bought those other games during that year.

My choice (as I said earlier) is Endless Ocean 2... now I personally only managed 115 hours so far on the original, but phisheep has clocked up almost 900 hours on it, which could certainly fit nicely into a years gaming. (it's about 2h 20 minutes per day)

However the year does not neccessarily have to be filled with playing 1 game alone, the question was if you could only BUY one game in 2010. You don't even have to play the game you buy at all, or you could complete the game you buy but play other older games too.

Gran Turismo 5. Runner up Fallout: New Vegas.

Both those games will last a looooonnnnnggggg time.

Hmm i would pick Gran Turismo 5 but if FF13 versus is out then i would buy that and rent GT5.
GOW 3 is close behind GT5. hell i'll rent all the games of 2010 since i can't choose.

heavy rain

Final Fantasy XIV.

GT5