Here, I'll start. When I was 8 years old, my parents went to a dinner party and plonked me down in front of the host's computer so I'd stay out of their way. The game they booted up to keep me occupied was Space Quest II. Little did they know what impact that would have on me...
My sister and I used to play as much as we could from Purple Moon: Secret Paths in the Forest, Rockett's New School, and so on. And look at me, now I'm working towards getting the Rockett games supported in ScummVM, haha. So, if I had to pick one to answer the question, it'd be Rockett's Tricky Decision which got me into adventure games.
Putt-Putt and the various Disney Animated Storybooks were lots of fun, too, I have fond memories of those. (As you can see, I grew up in the era of 1990s CD-ROM edutainment)
But the game that made me realize the true potential of what you could do was Riven. I borrowed my friend's CD-ROM set when I was in 6th grade or something (a box of 3 or 4 discs, that's how big the game is) and I was like, holy crap, this is better and even more immersive than Myst. Riven is how I fell in love with the whole Myst universe.