I suppose Super Mario Bros. for me. That's where I was at an age where I realized I really loved playing games.
My first game was a Pacman clone called Rat something. You were chased by cats, but if you grabbed an item you would turn into a dog and hunt the cats. I found out how to lock myself into a room so I could have a break.
I was only 3 or 4 years old, too young to realize I was into gaming.