For me, the easy answer would be Leisure Suit Larry. Oh boy, I remember having to explain to my dad why I'd written "prophylactic" on a piece of paper near our communal PC. The answer -- I wanted to look it up in the dictionary (we didn't have the internet back then). Sufficed to say, it was an awkward childhood moment. The less easy answer? Neuromancer.
A point-and-click adventure game based on the William Gibson novel of the same name, it was my glorious introduction to cyberpunk. I can't remember exactly when I played it -- definitely sometime in the early 90s -- but I know as a younger soul, a lot of the flavour was lost on me.
As far as I was concerned, I just wanted to sell my organs as quickly as possible so I could buy a neat cyberdeck, only to realise the loss of constitution would transform virtual me into a total wimp. I know now that the world of Neuromancer was much more complex and rich than my mind could understand at the time and as a result, my memories of it are vivid, yet oddly superficial.
Still, I remember being entranced by the concepts of wetware, ICE and cyberspace, which would many years later have an influence on my own games -- the visual design of the AI watchers from Deadnaut is a good example.
I also never completed the darn thing, so that's another reason to revisit it.
What game do you feel you should go back and tackle, now that you have more years under your belt?