For me it has to be the Call of Duty franchise.
It's a generic military shooter, it has regenerating health, the game is geared towards online multiplayer (I never play online multiplayer on any game), the story is always incredibly boring, and all you ever do is walk from corridor to corridor slowly shooting enemies and occasionally hiding behind cover to heal.
And yet, despite all of that, it's FUN to play. Proof that good game-feel can make even a hater enjoy it.
It's no TimeSplitters in terms of multiplayer or Deus Ex in terms of singleplayer, but the Call of Duty franchise (especially Black Ops 2) is damn fun to play.