Upper Brinstar, Super Metroid.
I had just moved to a new state in the US, and I didn't have any friends who played games for a while. Eventually I decided to sit with the "loser kid" at lunch, and I found out he played games too, and we became best friends for the rest of elementary school.
One of the first times I went to his house, we played through Super Metroid. It was the first time I had played the game. I fell in love with the atmosphere of this game, and I distinctly remember watching my friend play through Upper Brinstar, exploring the overgrown caverns and getting the morph ball and opening new areas with the missile expansion. It was at that moment that gaming was cemented as a social hobby for me, and I realized that I loved sharing my gaming experiences with others.
What about you?