I would say:
Resident Evil 6. Really enjoyed the game for a good portion, then it became a slog. While the story and explosive elements lead to shock and awe moments, by 3/4 of the way through I wanted the game to end. It just goes on and on and on. The parts that really grated, were replaying the ogre man fights multiple times with various characters and endless QTE segments. The segments were poorly designed and didn't leave anything other than a bitter taste.
I was most let down, as RE is and has been my all time favorite series since the PS1. I've played and typically enjoyed all of the mainline entries (0,1,2,3,4,5,CV and Revelations). I'm omitting 6, because while I didn't love 4 and liked 5, but consider it not really RE - 6 feels like RE, but is a far less enjoyable game when finished than 5 was. That was the disappointment. While I liked it more than 5 initially, I was done by the end of it. Revelations is without a doubt the best RE game this year and one of the better installments, in the series.
I've already traded in 6 and RE-acquired Dragons Dogma. That game is love.