Difficult choice to make...

I definitely know that Oblivion and Mass Effect would be in the top three (not going to place one above the other; both games are a masterpiece in their own right), however I'd have a difficult time picking a third. It would likely be a showdown between Rock Band and BioShock (I will admit, I've owned BioShock since it came out and have only played about an hour of it, if even).