PC Gaming

Dungeon Keeper I would say. Possessing a fly for the first time what quite a surprise.

Shogo: Mobile Armor Division by Monolith Production, before they made moved on to only making Middle-earth games they made some very interesting games, and Shogo was one that I fell in love with. The thing that really blew my mind, is that when you are in the mech parts of the game, which looks and feels like the same first person as on foot but with mech walking sounds and mech weapons, but then suddenly you see teeny tiny humans that shoot at you (sometimes with rocket launchers), literally shrunk down, you could even step on them lol. (and the blood effects on them were a bit comical considering how small they were)

The first 3d(not top down) GTA game...the freedom to explore was epic

Has to be The Sims. I remember going into Toys R Us with my best friend and we each picked up copies of the base game with the Livin Large expansion (remember toy stores and when you had to go and pick up physical copies of games?!)

Say what you will about The Sims but it was my first real exposure to just how expansive, creative, and immersive games could be. The possibilities felt endless!

Sim City - I loved that on my Amiga and played it to death.

Also Secret of Monkey Island, first game that was fun and funny - opened my eyes.

Ooh the first game that really blew my mind on the PC was Homeworld Cataclysm. I spent so many hours as a kid just going back and forth in the tutorial section of the demo, making a huge space fleet with no pressure, just gawking at the beautiful space backgrounds, the cool spaceships and their visual effects of thrusters, lasers and beam cannons.
It's still one of my favorite strategy franchises and games of all time.

Probably Rome: Total War in 2006... After being used to normal RTSs, the scale and complexity of the battles in RTW blew my mind, I couldn't understand how it was technically possible.
Still a fan of the series so many years later, so great.

For me I'd say it was probably playing Carmageddon 2 at my friend's place. I think that was also the first time I played a 3D-accelerated game (Voodoo).

Myst has always stuck with me as looking amazing at the time. I totally sucked at the game but its graphics have a fond place in my mind for some reason!

Dungeon keeper 1. Real time strategy and first person shooter at the same time. Its like playing strategy game with a view from the top and then you can switch to Minecraft mode, dig the walls, build things, fight enemies.

really dating myself here, but I could not get enough of the OG Sims game. I used to play until I fell asleep at my keyboard. It felt so innovative at the time.

Unreal. At the time Quake II was the most popular FPS and the colors and atmosphere of Unreal really were a new experience. I remember the graphics card wars at the time too. Really fun times for computer tech.

Half Life

Maybe too retro for "PC" gaming, but Ultima III on the C-64. To have a huge explorable world full of details and people to talk to sucked me right in. Others did a better job later in graphics, size, and details, but Ultima was the first.