Sandbox games require the player to navigate an environment without having to move through it level by level. The "open world" often connotes a non-linear approach to objectives and allows for free roaming, encouraging exploration. As processors get better and better, a game world may become larger and larger, moving away from text based narratives (which were technically open world permitting many actions in a world entirely within the players imagination), towards a more expansive, sprawling setting. For the purposes of this article, the use of "largest" does not refers to simple physical sized but to the worlds I found to be organic, in which every inch breaths and feels alive with spontaneous events and believable characters.
5. Minecraft 
Although theoretically the largest map on this list, I have placed this entry fifth because "infinitely generating" is cheating. Still, with an infinitely customizable environment with its trademark pixelated graphics, Minecraft allows you to explore, create and discover in ways entirely open to any player of any degree of computing skill. If you want a survival game then dig in and survive the creepers that emerge at night. Or you can create a roller coaster and soar through he sky. With any number of mods at your disposal you can face dragons or build structures from the deepest recesses of your mind, or simply carve out another Helm's Deep.