#1 Minecraft
Minecraft is by far the largest world you could ever visit, thanks to its seeded (or procedurally generated) environment.
Every journey you take in Minecraft is an adventure seeded with numerous challenges (in the form of angry skeletons or exploding creepers) and rewards (like diamonds!). It's possible to adventure alone, or with a friend—or many friends, for that matter thanks to the game's flexibility.
Each of the adventures you have can be every bit as memorable as any in a pre-designed world.