For me it's definitely Call of Duty Black Ops 2.
I have so many hours invested in its online multiplayer portion. The new version of Nuketown was crazy when they made it live 24/7 or when they increased the player number in a lobby. The only horrible experience I've had was from hackers. I didn't mind if they hacked the lobby to make the player slots larger, but when they made themselves invincible or gave themselves aimbot, then we had a problem.
Darth Maul got a reaction from DC in Best FPS Game of All Time?