The Elder Scrolls V: Skyrim

You are the Dragonborn, a chosen warrior who can absorb the power of dragons, in a snowy province on the edge of civil war. The story is the least of it. What people remember is the freedom: climb that mountain, rob that house, join the mages or the assassins, ignore the main quest for fifty hours. The world is open and forgiving, easy to pick up, and stuffed with caves and ruins and small towns to wander into. The systems are shallow if you look closely, but the sense of a whole snowbound country waiting to be explored has kept people playing for over a decade.