Probably Hyrule Warriors. Got bored of it real quick.

Halo 5 for sure

Rock Band 4! It is a bug filled mess with less features than RB1, from 2007. After 6 months in the world, there are still basic problems that are unfixed. Guitar Hero: Live tried to do something different, but that seems to have put the nail in their coffin too. I guess the plastic band game really is a blast from the past now.

Tony Hawk, was a completely different thing when was announced, so this is Disappointing.

Batman Arkham Knight, not a bad game but it was dissapointing.

whatever your favorite game was

how so? 3DS sales are still pretty damn solid on a weekly basis for a system that is getting minimal software releases recently. Its got a good library and was well worth 200$ (IMO).  I do doubt that the next Nintendo handheld iteration will have 'DS' in the title or will be a similar concept (I think more likely it will have one dedicated screen and higher specs), but we'll see

as far as disappointments, #1 by far: Fallout 4. Its not a bad game per say, and as for open world action games if it weren't a "Fallout" game I think it would seem fairly good but in comparison to its predecessors something is missing. It lacks the choices and wackiness of New Vegas and even (somehow?) seems to lack some of the weirdness and quest innovation that Fallout 3 had. Somehow everything in it seems oddly... the same? like after 20 hours of playing the game you've come across pretty much everything 'new' that you will for the next 40 hours :/

I also find it shocking that a brand that prides itself on 'freedom' of the open world and characters, that everything would seem so black and white and boring.

The only faction options are all seemingly fairly dislikable and even then only one is a truly logical one to join (the dictator state BoS), certain factions have literally NO variety or interest (for example its hard to get immersed in a game when EVERY single raider in the game automatically attacks you no matter what you're wearing or what you do, it would make it far more interesting if there were some gray area 'bad' characters in the game in terms of what happens when you approach them out in the world), the added voice and dialogue options are terrible, the 'romancing' option in the game is poorly done, and frankly you're not really given much of a strong reason to care about the main quest in the game and somehow its paced and crafted so poorly that you forget what your main objective is for long periods. Limited amount of choice hurts the game too, it almost feels like a rail game in the sense of forced plot (very much unlike New Vegas). Lastly arguably the worst thing about the game is that they added the clunky Mine-craftesque community building- it absolutely distracts from the world and kills any and all immersion. Your communities have essentially no bearing on the rest of the game and its hard to believe that in such a ravaged world you would be able to so simply put together a community chain bigger than anything else in the game that easily. The communities you build have like no personality and, again, absolutely destroy immersion into the game and end up just feeling like a pain to go build up and defend against generic AI that spawns to attack them.

rambling sorry.

other disappointments in my mind include the 3DS Mario Party game (awful board setups), Dragon Age Inquisition (better than Dragon Age II, but dramatically worse than its original with boring grindy gameplay and a dead seeming world), all of the Mario 3D games for this generation (hate to say it but a good Mario game needs to have its own 'feel' and a somewhat immersive world, what made entrys like Galaxy and Sunshine refreshing was they had such an identifiable world of their own), etc.

Pokemon X/Y
This was easily my least favorite mainline Pokemon game to date. The story was awful. The new Pokemon with the exception of like 2 were all bad and there were very little new Pokemon introduced. I really wasn't a fan of mega evolutions as well. There was pretty much zero post game content. Team Flare was also by far the lamest of all the villainous teams. I could go on about how much i dislike this game.
The game did do some things right though such as character customization, having an aesthetically beautiful world, and having great online features with the PPS. The only reason i kept coming back to this game was because of the PPS and this is what saved the game from being a total disappointment for me.

Well yeah, if you wanted or cared about Tony Hawk. If people didn't care about the game, nor have they played it or any subsequent games then they weren't disappointed by it. That is probably the majority of people on this site. 

Watch Dogs. Everything about the game was decent to good, but that's all it was, decent to good. It was just a slightly above average game after the previews made it look like it might be a decent contender against GTA.