I think it was Pac-Man when I was about six years old. I remember going to the arcade with my father, and he just kinda let me play it. I don't remember it that well, but I'm pretty sure it was that.
Then there was also the first Super Mario Bros. on the NES. I remember that one a bit better. Probably because I was a bit older. It was my first home console game. I remember playing it a lot and later even beating it. I was eight at the time, and really, the game wasn't all that hard for me. I hear people saying how difficult it was even when they were kids, but to me, it really wasn't once I beat it. Well, except for 7-4, which was really confusing, and took me a while to get through. I did manage to figure it out all on my own, though.