Pokemon, because it has infinite replay value, especially if you include romhacks that do things like expand the amount of mons you can catch in in one region + the difficulty of NPCs/bosses (because tbf the original games are mostly pretty easy and can be completed with brute force and not much strategy, grinding is usually the worst part but they made grinding easier in later entries).
Halo because of the unique art/designs, the atmospheric kino soundtrack, the fun/satisfying mechanics and level design, the highly customizable multiplayer experience, and the story that has actual depth in how it approaches subjects such as religion, holy wars, the afterlife, genocide/extinction, enemies becoming allies etc.
Gears Of War for similar reasons as Halo--great art style/music/atmosphere, hands-down best third-person cover mechanics ever, and considerably skillful multiplayer that greatly rewards teamwork.
Call Of Duty because it is accessible, addicting and satisfying. Its the kind of game where no matter which one you are playing, you can just hop on, join pretty much whatever gametype you want, use whatever weapons/loadout you want and have fun. You don't have to be good at it to enjoy it, and that's the whole point. The progression and unlock systems are basically the best of any online shooter as well.