Super Mario Bros. on NES. My mom tells me she desperately tried to talk my uncle out of buying me and my siblings a game system for christmas, but he wouldn't relent. I remember we couldn't beat even the first stages (I was 5, I think?). My mom, despite the protests, was playing with us and was the first one to get to world 1-3. After some panicked yelling about not knowing what was good and what was bad, she defeated the bowser. We were super stoked because mom beat the game! Of course, Toad informed us the princess was in another castle. Mom came home from work the next day with the shocking revelation that there were 8 worlds to get through.
While I didn't get heavy into gaming until we had the genesis we did always regret selling the NES. I've long since replaced it, although I don't know if I have all the same games... I know we had Where in Time is Carmen Sandiego, but I haven't replaced that yet.