I played Tetris 2 with my older brother so much that I started lining up things in real life in my head.
'That's red, it goes over there, yellow over there, blue turn it sideways over there.' That was seriously a nightmare. Oh, and Street Fighter 2. We had Street Fighter marathons like nobody's business. It has been forever since I stayed up all night playing a game. Fallout 3 comes the closest to my obsession days.