There isn't any one game.
When I first started playing, every game I played stayed in my mind, starting with Myst and stomping around Channelwood.
Then Shivers with its weirdly intense colors and strangely eerie music.
Byzantine with its real life Mosque and cistern and computer generated views of how they looked in the past.
Drowned God with its sad story and creepy oppressiveness.
Obsidian with its Magritte-like gameworld -- the workers with computer monitors for heads, the room where you had to walk up and down the walls and ceiling, the wave puzzle, the spider puzzle...
Ripper with its weird puzzles inside a computer and an over-the-top performance by Christopher Walken.
The rat catcher in Atlantis -- a strange respite from being killed or captured by guards every time you turned around.
Atlantis 2 with the Irish horse, the blue guy in the sky, and the dragon swimming around the aquarium.
9: The Last Resort with its weirdly colorful gameworld, featuring Aerosmith members as giant twin worms and Cher a-moanin'-and-a-groanin' every time you saved your game.
Armed and Delirious with it's giant toilet, marching rocks, and flying washing machine.
That strangely atypical moment in Ark of Time where
your character stops being a jerk for once as he reads about the sad fates of the members of Atlantis who'd been travelling abroad when it blew, and were stranded in primitive cultures.
Liryl the lonely guardian in Lighthouse.
Learning about the wharks in Riven.
The enormous living ships in Schizm.
The fairytale-like night garden of Brada Coe in Schizm 2.
The music in the lighthouse, the sound of the ancient computer booting up, and the strange animals in Amerzone.
Following the vines and their glowing seed pods in Edanna in Myst III: Exile.
The story in Moment of Silence, and how it makes you wonder about how easy it is to manipulate and erase digital versions of things in real life.
And two of the best were Morpheus and Faust: 7 Games of the Soul, which spawned some great discussion posts about hidden meanings, ***one***
of which is archived. Unfortunately the Morpheus thread was lost.