From a pure game standpoint, I'd probably say Dishonored. It's not just got the perfect gameplay, it's connected in a good way, the setting perfectly lines up with I like, and it has some great background that you can pick up while playing. Add in a legitimate open world choose your own way to beat a mission style gameplay, you actually have quite a few ways to play through the game.
I'm willing to overlook the ending nonsense, because the game's just that fun.
Though, nostalgiawise I'd probably say Halo 2 or Battlefront 2. I remember so fondly the times when I'd wake up at the rich family friend's house early just so I could play Battlefront 2 on my own, or when my brother and I would plough through Halo 2 on co-op legendary. Or all the times we four brothers played Shotguns-Swords on Lockout.