It really depends on your definition of "disappointing."
Obviously, that definition depends entirely on your expectations concerning a particular game. So, if we measure overall hype against the final product, could we argue that Star Wars: Battlefront is the most disappointing title of 2015?
It's hard to say. Maybe a lot of people weren't expecting a critical smash, so they won't be surprised at the merely average Metascore . On the other hand, I think many were anticipating a more robust and rewarding game out of the gate. I'm not saying everyone feels the way Benjamin Burnley does but I've spoken to lots of shooter and "Star Wars" fans and even those who like it sort of have a "meh" attitude. "It's fun but it's nothing special" is really the summation I'm hearing these days.
Otherwise, I think a lot of the hotly anticipated 2015 games did exceedingly well. Bloodborne is considered by many to be a legitimate Game of the Year contender, Call of Duty: Black Ops III was a generally well-received, rock solid shooter, The Witcher 3: Wild Hunt definitely had a troublesome launch but the game itself is a gem, everyone seems to love Fallout 4 , and there's no denying the greatness of Metal Gear Solid V: The Phantom Pain (even if some fans still say it doesn't feel like MGS). Even Assassin's Creed Syndicate turned out great, which was probably a surprise to some.
So I guess it really is Battlefront , although I imagine many will vote for The Order: 1886 , which I don't agree with even remotely but I won't rehash that argument.
Related Game(s): Star Wars: Battlefront