Well in the sense that it's the first game I remember my Dad owning then I'd say MGS. I'm sure we both completed that game about 50 times in total, always trying to save Meryl. It was only recently, about a year ago, that I actually figured out how to do it. Man, that was one of the longest challenges ever.
EDIT: It was actually probably a pack of SEGA games for the Genesis. It had Sonic, Monaco GP, Streets of Rage and Shinobi. These games were the ones that shot me into gaming at a young age. There we go, bonus points for it being made before I was born.