Of the ones I've played so far? Well, if Batman came out this year then that one definitely is up in my top 3 list, possibly my number one even. Portal 2 also is very much loved for the same reasons everyone has already said. As for the third pick for me, I'd go with the much under appreciated Dead Nation. Admittedly, it's not the most beautiful to look at nor the most compelling story, but it's got fantastic gameplay when you have with another person in the room busting zombies with ya. And damn it if there isn't another good 2 player zombie survival game out there. (RE5 doesn't even really compare to this one in my books.)
So my quicky list is as follows:
1. Batman: AA
2. Portal 2
3. Dead Nation