The Elder Scrolls: Daggerfall was a complete mindfuck back then.
Being able to walk, ride or fly through a continent the size of Europe, in 3D first person view, with different climate zones and seasons, buy a ship, turn into a vampire or werewolf, enter any house on the map and have NPCs react to what clothes you wear…
There was nothing like it before, and its freedom and scope is unmatched till today.