Oh that's an easy one
Metal Gear Solid The Phantom Pain.
Not just the level of hype associated with the game but the whole saga surrounding Konami, Kojima and their pissing all over their key game franchises and that's before you even get in to the fact that the game is very meh at best, oh and the fact that pretty much every game reviewer was falling over themselves to felate Kojima for this game.
Where to start, well the story is weak as fuck, easily the weakest of any of the MGS games, but add to that a pacing and delivery of the story. Entire sections that are monologue boring, the fact that every mission had opening and closing credits (serious wtf was that about.) The attempt at grit and edgyness but in turn the utterly appalling potrail of Quiet and the limitations when you had interactions with the child soldiers.
The game was clearly half finished, with a large portion of the 2nd Chapter being replays of Chapter 1 missions with a difficulty twist
The entire FOB section was shit, I mean just shit, not withstanding that motherbase was dull, boring and utterly devoid of interesting features you had two levels of FOB infiltration, the one where you encountered no one or the one where you encountered an army that would happily take on god... and probably win. Oh and that the whole thing was a requirement if you want to unlock the higher end development projects without grinding like a five dollar hooker for most of your life and that's BEFORE you add in the whole microtransaction, FOB extension insure your base items bullshit.
The AI was just awful, ranging from the terminally baffled to the guy who could see you taking a dump in motherbases toilet 3 hours before the mission kicked off.
The bosses where easily the worst of the saga, not withstanding the fact that we had yet ANOTHER psychic, skull face was just boring and the fire guy, well I can't even remember his name that's how bad he was.
No doubt their is a lot more I could mention but given this was easily one of the biggest titles of the year, that it had so much controversy surrounding it's release and the fact that the game as a whole was utterly mediocre I think it stands out as the biggest let down this year.
But at least the PC port was good... Dark Knight I am looking at you here.
I just hope that Fallout 4 doesn't suck the big one because if it does then it will take over.