Outer Wilds. A lot of games have a high level of replayability, but your first playthrough of Outer Wilds is something you never get back.

Portal.

I'll never forget the day I started playing this game. It was so cool and so different from anything else I had played that I couldn't stop playing, and finished the game in one session. Awesome night!

Mass Effect 1.
The first encounter with the Reapers, with Sovereign, is such an incredible moment. These horrifying, unkillable deities that can't be reasoned with or stopped that just want to destroy was so intimidating on the first playthrough.
The reveal about the mass relays and how they guided tech advancement was incredible too.

You exist because we allow it. You will end because we demand it.

Ultima Online maybe.

The Mass Effect Trilogy, no hesitation

Half-Life, hands down.

I'm going to say 3: Final Fantasy VII, Resident Evil and Subnautica.

Definitely fallout New Vegas, just amazing game.

Horizon Zero Down. My favorite game ever.

Oblivion. As much shit as it gets, I think it's still the best Elder Scrolls game to date, with Morrowind as a close second.

Probably Deus Ex.

I'm stuck between three picks.

Morrowind, Tony Hawks Pro Skater, and Kingdom Hearts.

Easy, the game that started it all for me - Zork.

The Last of Us 1 & 2

Subnautica, easiest answer for me!

Bioshock and Silent Hill 2

Edit: also a very old and very obscure PC game called AMBER: Journeys Beyond. Basically, you had to go check on your collegue and do some ghost hunting and puzzle solving to save her

The first (and best) BioShock. I have no idea how I got away with so many years of not knowing the story or nothing being spoiled, but man, playing that for the first time was an absolute trip, especially that part.

Chrono Trigger.

I get to run through all 13 endings again? Experience all those plot twists and storylines? Yes. Please.

That first playthrough on New Game + was magical.

Portal 2

Super Mario Bros. 3

Factorio. I had so much fun figuring out all the stuff it throws at you and making a mess of spaghetti beacuse you couldn't predict what was next to sort things out.

Would have loved to play some of the early 360 games online including Halo back in there prime. I just didn't have good enough internet for it to be playable back then.

Oblivion. Probably sunk 1000 hours sucking every last drop of dopamine out of that game.

Factorio. I had actually become aware of how fun building games could be through Prison Architect a couple years prior. But Factorio took it to a whole other level. Extraordinary freedom and near limitless things to do -- it's distilled, weaponized, dangerously addictive fun for people who have engineering brains

Star Fox 64

Bioshock, more importantly, that scene

RDR2. Still the only game I ever took a day off for on launch day.

Celeste, because dang are those platforming controls so good.

Final Fantasy 8. Loved the game

Super Metroid

Halo 3 back in its heyday without question. The hours and hours spent grinding ranks in playlists and playing custom games is some of the best times I’ve ever had in gaming.

Knights of the Old Republic

If I could also not have played Half Life, I'd like a do-over of Black Mesa. Don't get me wrong, HL is still brilliant, but the Xen section of BM is just chef's kiss

Dark Soul 1, especially finding new shortcuts and new hidden things

Okami hands down. This is the only game that I've bought for multiple consoles. The art style and solid game play age beautifully and the game is still as much fun today as it was when it first came out.

There are many, but I think I would take Morrowind, it was magical as a kid.

TLOU

Life is Strange.

The Witcher 3. Still love it after multiple playthroughs, but miss the feeling of exploratory wonder when I didn't have the map memorized.

Red Dead Redemption. When the music starts picking up as you enter Mexico for the first time it was beautiful and something I had never experienced in a video game before.

Metal Gear Solid!