For me, my list is thus
Final Fantasy XIII
Really took a step backwards in many ways. Story, complexity, open world. X and XII were bound by borders, but nothing spoke "constrained" like XIII. For me, XIII was the game where I felt Square-Enix focused too much on what their audience wanted to see, and not what their audience wanted to play. I used to buy Final Fantasy games pretty blindly with collector's editions at midnight releases. XIII made it so that I scour reviews and game-play FIRST. I still like Final Fantasy games... but my faith in them "just being good" out of the gate is forever tarnished. I'm praying to the old gods and new, the known and unknown -- that they don't screw up VII's remake.
Command & Conquer 4 (Tiberium Saga)
Command & Conquer Red Alert 3 (Red Alert Saga)
EA Games. The grandfather of RTS games appealed mostly to base builders. C&C4 removed that audience along with some very very questionable story changes. C&CRA3 was 99% pure satire, and lacked the charm of Red Alert 2, instead becoming a big mockery of that branch of C&C games.
Assassin's Creed: Revelations
Adding a small change to parkour, or a different assassination animation, while still casting us as a white(ish) male lead in a different setting IS not a good basis for new games in the franchise. Revelations and forward; I just felt like everyone was Ezio or Altair. Just re-skinned. I think 2 things really broke Assassin's Creed for me. 1. Recycling content. 2. releasing games too regularly. I mean, how many AC games are there now that could be reskins with small tweaks? 6? The recycled content just makes everything so boring, and the frequency of releases made it all feel like a huge cash grab. Also, the PC ports are why I no longer buy Ubisoft games for PC any longer.
Pokemon: Ruby/Sapphire
(and then) Pokemon X/Y
Ruby and Sapphire showed me that fans will never get a game (or I guess 2 games) in which you would be able to experience an full pokemon world. You won't be able to actually go out and visit other world areas. You won't be able to catch ALL the pokemon without jumping through hoops (more and more hoops as more games were released). You won't get to dictate what sort of trainer you will be. It was always going to be the same recycled formula. Same starter types. Same path through the gyms. Rival & Championship. And as you probably know by now, I hate recycled content. I guess Black & White/Black 2 & White 2 tried to shake up things a little -- but it still suffered under the weight of recycled content. Then X/Y brought Mega Evolutions, which was about as gimmicky as gimmicks get -- so that was the last nail in that Koffing for me. I would of loved to also see the reason for it being difficult to raise pokemon of different types. Everyone is always so amazed that you can raise more than 1 type of pokemon to become champ. Type affinities that benefit you/the type that raise or lower based on training would have been a welcome addendum to the experience gain system. But I digress.