Oblivion.

I think I was 12 or so when I first played it and absolutely no other moment in gaming in my life beats the pure joy and ecstasy I felt when, on my second start of the game, instead of leaving the sewers and going straight to Vilverin, I turned around and found the Imperial City and ran to it, being amazed that I just could do it and the game just let me do it. It was the first time I understood the concept of an open world game.

My first start of the game I went straight to Vilverin and couldn't go down the first flight of stairs because I was looting everything and I got over-encumbered. I'm not a native English speaker, and at the time I didn't understand what that word meant, so I just thought I didn't something wrong since I couldn't move or do anything and just decided to start again.