My vote goes to Ace of Spades. I know that many people will say Halo 4 but the original Ace of Spades was legendary. It was a free indie game with a similar map format to minecraft (but all blocks have the same "health" and are only separated by colour. The rifle killed with one shot to the head so people built trench systems, fortresses etc to survive and undermine the enemy. Also if you destroyed the base of anything it collapsed so digging was a key part of the game.
Then JAGEX game along and bought the title. They introduced CoD classes, rocket launchers, sniper rifles and the jetpack. These editions made trench warfare and fortresses pointless because you could simply fly over them and thus, the game was completely ruined. You can still play the Classic version but the developer is no longer aloud to update an original game.
[b]TL;DR[/b] JAGEX bought the franchise and destroyed everything original about it, turning it into a crappy CoD clone.
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Most recent for me would have to be assassins creed 3 and possibly revelations. Seems to be leaving stealthy roots and just going into the action pirate home it's created. This could all just be fatigue of the series though considering its been one after the other for the past three years.