Toss up between Punch-Out on the NES and Street Fighter II on the SNES.
I remember once in grade school I got sick at school and my father had to pick me up. Some kind of stomach virus or something, I can't remember. My father stopped at a pharmacy for some medicine which happened to be right next to this video rental place. He goes into the pharmacy, comes out, then goes into the video place. Comes back with some medicine, and Punch-Out for NES. I remember as soon as I got home, I played that game the entire day until bed time. And even though the next day I was feeling much better, my father made me stay home from school in case I got sick again. So I played it all day that day as well. It was the first time I ever felt the desire to play a video game that much. The first time I ever enjoyed a video game that much.
And Street Fighter II, well...I guess it was just a monster arcade game that I, along with most of my friends, fell in love with. So when it came out on the SNES it was like a dream come true. We were always trying to coordinate sleepovers on weekends so we could have SFII tournaments and such, each person sticking to one character. Mine was always Ken. Playing SFII was also the first time I really started paying attention to the music in games.