Gears of War 3 should easily take this when all is said and done but there's been plenty of awesome co-op and more to come.
I'm thnking that Uncharted 3 is going to outduel Gears of War 3 in the co-op arena. Gears is great, no doubt, but Uncharted has never had campaign co-op before so the freshness alone of the 2009 GOTY should take the crown.
Uncharted 3? has campaign co-op!!!!!!!!!!!!?