The Legend of Zelda — The Gold Cartridge and the Open World
Battery saves, a gold cart, and a world that let you wander.
That gold cartridge meant something. The Legend of Zelda (1987 in the US) was one of the first console games with a battery-backed save: no more passwords, your quest was yours, waiting where you left it.
Miyamoto described it as a "miniature garden" you could explore in any order. Burnable bushes, bombable walls, secret rupee caves. It rewarded experimentation in a way nothing else did, and it shipped without a map, so we drew our own on notebook paper.
"It's dangerous to go alone." The sword on the cave floor. Hyrule's overworld theme. This is the cartridge that taught a generation that a game could be a world, not just a series of screens.