It would be Mega Man. I watched my uncle play it around 1997, and it was the first contact I ever had with games. I would watch him play the whole day. I was impressed by the level of difficulty the game would present, and how you would lose everything if you died a certain number of times. Even though it was hard and I wouldn't get to play much, I loved it. And then, when I first played it, I felt great, because I learned to play it just by watching, and my coordination was pretty good for a 4-5 years old boy who never touched a gamepad before.
To this day I'm still a fan of the Mega Man series, from NES games to the next-to-release spiritual successor Mighty Nº9. And also in love with games in general.