Portal 2 as much as I love it shouldn't be on this list. Once you solve the puzzles there is no reason to go back to it really. At least the other 3 you can revisit and have some fun.
Gears of War 3 although has some solid ideas they are really hindered in the longevity department by not being a modern military shooter. its fun as a distraction but there isn't the depth of the other two.
MW3 has the full features set of a CoD game, has some refinement a few changes and continues to offer many players hours of enjoyment, but there is no risk taken and its really the same thing in a different box.
BF3 shouldn't win either but its the best of the worst. Its the only game out there on the list offering co operative gameplay in a competitive environment. Definitely a step up from most games out there.
My pick is Dark Souls. This might be hard for some people to wrap their heads around but its the only game this year where I don't go into a mode to play it. I am always playing both.