Hmm... Myst, I guess. I also beat several Sierra games and 7th Guest without any hints back in the pre-internet days, but Myst stands out as being the hardest.
(Well - I do remember a Sierra operator gave me a small hint when I called up to order a King's Quest game and mentioned that I needed it as a diversion because Myst had me stumped. Does that still count?)
I also remember being very proud to beat a text adventure, Mission: Impossible, all by myself at age 9 or 10. Not a complex game but you died a lot, very punishing. I'm not sure I could do it again today.
