Assassin's Creed III
I was one of those weirdos who was actually invested the on-going meta-plot involving Desmond, Abstergo, and the Assassins. After all the intrigue, mystery, and set-up, the ending was just sort of...eh. Not terrible, not great, just sort of there. It did it's job, but it was really abrupt and didn't really satisfy. And it most definitely wasn't worth all the build-up and anticipation. (Heh, heh...)
On the historical side, the setting and plot were just "okay" compared to previous outings. As an American, colonial America was just too familiar to be really interesting compared to the Crusades, Renaissance Italy, or Constantinople. And as a history teacher, the idea that the same conspicuously-robed Native American assassin was at the signing of the Declaration of Independence AND the Boston Tea Party AND the Battle of Bunker Hill AND had face-time with several Founding Fathers and this wasn't mentioned in any history book? Sure. And then there's straight-up historical error: the Boston Tea Party happened at night and was performed by the Sons of Liberty who were dressed as caricatured versions of Mohawk tribe American Indians...not in broad daylight by random rebels who were cheered-on by people on the pier while Redcoats stormed the boats and were brutally killed by a conspicuously-robed Native American assassin! Even taking into account "the history books are lying to us" conspiratorial angle of the series, it was still lame. Again, I'm an American and a history teacher who spent years teaching early U.S. history, so the statement "familiarity breeds contempt" definitely applies here. I actually really liked the parts that deal with how Conner relates to his tribe and the other non-historically-based characters, but whenever the story brushed up against real history it just came across as silly. How much of it is "familiarity breeds contempt" and how much is the actual game's fault is arguable.