This is going to be a VERY obscure reference. But I just finished playing Adventure A: Planet of Death, a 1981 text adventure from Artic Computing for the British Spectrum computers, and was pleasantly surprised by it. There was more content to the game than I expected, and even though the parser was obstinate, there were spelling errors galore, and several of the puzzles felt unfinished, as though details were planned but not implemented... I still had a lot of fun with it.

Just wandering around the game world was pleasant; the puzzles were sensible and not overly difficult, but there was enough to experiment with that I felt free to enjoy the sense of discovery. As poorly executed as some aspects of the game were, I ended up having a lot more fun with it than I thought I was going to a few turns in.

Myst. I had already played AGs - like Fools Errand and Uninvited - but Myst was so totally different.

It was mid 90's, and my (then) boyfriend gave me a mac-CD and said I think you will like this. I had no idea what so ever what this game was about, I just ended up on this island, not knowing what to do.
I loved every second, and I don't think I will ever feel the same way again. Nothing like the first love.

Overclocked has surprised me the most. The way the story is told is a refreshing take on now narratives in games can be told. Post Mortem and Still Life are close runner ups with the Art of Murder games close behind.