Has to be God of War: Ascension or/and Gears of War: Judgment.
Even though both games have sold quite well and GoW:A is showing great legs, both God of War III and Gears of War 3 sold 4-5m each by the end of their first year and over 6 million units ltd. So even though games like Remember Me, Fuse and Lost Planet 3 sold so very poorly the huge fall from grace by both GoW's is more disappointing.
At least GoW:A will finish at ~3m lifetime, which is respectable by GoW standards (not including GoW III) and still enough to be considered the juggernaut of the genre.
GeoW:J will finish at ~2m lifetime, this is not respectable by GeoW standards and considering it's a shooter, not impressive at all.