So many, it's hard to choose just one, or even just a few.
I guess I first have to go back to the Commodore Amiga, that's where I played some of the games I have so much fondness for. Things like Eye of the Beholder (a primitive version of something like Legend of Grimrock, set in the Forgotten Realms world), and... well, one of my favourites was little more than a bunch of blocky pixels on the screen, called Treasures Of The Savage Frontier. Another D&D game, set in the Forgotten Realms world.
Another was a game called Lords of Chaos. It was a very, very simplistic game about summoning creatures as a wizard, to do battle with the summoned creatures of an AI wizard.
Oh, and Lemmings. Who can forget Lemmings? So simple yet so insanely addictive. Were that made today, it would be an app, not a game, lol.
The original Elite. Hours and hours and hours spent in that. Amazing how it managed to create a universe you felt a part of even though it was very, very primitive, graphically.
Moving forward, I guess the game that defined my childhood was Secret of Mana, for the SNES. SO much fondness for that game. One of the best RPG's ever made, IMO. In a time where Square were lords of all they surveyed.
Oh, and the original Mortal Kombat. Only arcade game I ever beat. To this day.
It's hard to be nostalgic about games more recent than that, since they are constantly remade, or re-released, they're not hard to get a hold of. FFVII re-defined my view of what made a game great, although if I'm honest, I prefer VIII.
And Jade Cocoon. To this day I have no idea why I was obsessed with that game. But I was, and then some.