My choice is low hanging fruit, but Destiny. Jeff Gerstmann's rant about it is legendary.
But in short, everything was in place for Destiny to be the next big thing. At least as big as Halo if not bigger. Then management fucked it all up when they decided 5 years into a 6 year project that they didn't like the story. They ripped that out and cobbled together a game with the parts that were left. Some of the DLCs were found on the disc, and the game was still using Destiny 1 alpha leftovers as late as Destiny 2.
The shooting is great, and the remnants of what lore and ideas that remain hint at something truly special. All it was lacking was content and story, both of which appear to have been cut in the name of corporate greed.
And the cherry on top, Activision is mostly blameless since they were very generous with contracts and let Bungie do their own thing. It was entirely a Bungie management fuck up.
Further compounding that is you have two loadouts. You either have a pea shooter and a disguise, or you walk in dressed for the North Hollywood shootout. And some story missions force you into combat.