Advanced Warfare, when they released of that silly shit in MP. I realized that the era of modern military was dead, and all CoD could do was try and emulate their previous successes.
AC Unity, when I realized that the only thing that made AC unique and fun, the parkour, was being done better by other games (and oddly enough, earlier games in the franchise).
Far Cry 4, when I realized that from this point all they will be doing is trying to emulate Vaas and they never will eclipse that performance.
GTA/Most open world games, when I realized that they are all essentially the same and driving around doing side missions doesn't interest me anymore.