Close between Halo Reach and CE, but I'll go with the former. Made me emotional more than a few times, and going back to more open environments allowed some variety to the campaign (multiplayer was a disappointment though).
Made a bet with LipeJJ and HylianYoshi that the XB1 will reach 30 million before Wii U reaches 15 million. Loser has to get avatar picked by winner for 6 months (or if I lose, either 6 months avatar control for both Lipe and Hylian, or my patrick avatar comes back forever).