As much as I like the gameplay of the series,
My pick has got to be Halo 4.
Its not BAD, per se, but very little in it is good. Sure, the tried-and-true formula stands up pretty well with fun (but frankly bulletspongey) enemies but by now its just OLD, and it seems to have taken steps back from Previous Halo titles. No dual wielding, and the only enemies in the game are the ones from Combat evolved. Don't tell me that the Prometheans are good enemies either, they're not. They have sparks of fun now and agian but the Knights are just teleporting bulletsponges and the dog-things are more annoying than challenging.
Also, its short. REALLY short. as in, 3-5 hours shorter than previous titles. That annoys me the shiiiit out of me, and the multiplayer just feels like Call of Duty with higher jumping and Battle-Rifles. Seriously, 343i? You couldn't do anything new with the multiplayer so you borrow from CoD? Sad. sad days...
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Other than that, its been a great year for games. X-com and Far Cry 3 being my top 2 favorites