3. Call of Duty
It all began with Call of Duty 4: Modern Warfare, when the series introduced a ludicrously addictive multiplayer suite that pretty much influenced every FPS game that followed. Why is it so addictive? Modern Warfare introduced the Prestige system to online FPS games; as you play, you level up, and when you get to the max level, you're allowed to Prestige, which resets you to Level 1, but gives you a different badge as bragging rights. Throughout the series, Prestige Mode has allowed players to pick up additional perks and incentives, and though a lot of players seem to stop once they get past the first Prestige, that in of itself takes most players around 15-20 hours, by which point the addictive nature of the gameplay has taken hold. Activision continue to toy with ways to keep players hooked up to Prestige 10, and going by the hundreds of millions of hours these games log every year, it appears that they've managed to enslave an entire generation of players to its instant gratification nature of play. With a frenetic pace, immensely satisfying array of weapons and some superbly-designed maps, it's no surprise that the franchise continues to thrive in spite of its many detractors.