Mass Effect trilogy.

Or maybe Bioshock.

Either/both.

Very late edit: Deus Ex. I still play it every once in a while and while the graphics are terrible, it's a lot better with GMDX and you get used to it very quickly. That game is still a masterpiece.

Earthbound. Even on my subsequent yearly-ish playthroughs, it's so easy to get immersed into the beautiful quirkiness of the world.

Easily Majora’s Mask

Edit: upon further speculation, Final Fantasy X is tied

Probably Planescape: Torment

Gothic

My first open world game, unmatched freedom to do what you want and to go where you want, exploration rewarding with handplaced loot everywhere. I lived in that game for a whole year.

Factorio of course, because that's what people already do. They start over and over again with ever bigger plans.

Doki Doki Literature Club. Because I got spoiled the first time I played it.

I wish I could experience Portal for the first time again.

Half Life or Command & Conquer for me

Monkey Island 2: lechuck's revenge.

Definitely Subnautica in VR. I didn't play it on flatscreen, only in VR, and it was the most immersive and amazing experience out of any game.

Either:

1. Mario 64 - seeing a fully realised 3d world for the first time was mind blowing. Great music, adventure, and fun.

Or

2. Morrowind - what a wild and rich world to explore for the first time. My best friend and I rotated shifts for weeks during school holidays. Great memories.

Freelancer

Baldur's Gate 2. Would love to re-live the feeling of wonder and suspense this game offered, although it'd probably help to forget about modern RPGs too, to keep with the nostalgia.

System Shock 2 or Thief: Dark Project. Those two were the games that impressed me most in my 40 years of playing games.

Honourable mention: Minecraft. The first few weeks playing that come very close.

Death Stranding

I still have dreams about that masterpiece of a game. There's just so few like it.

There's a couple I'd have to choose from on this:

Portal 1 and 2 (if it can only be one game and not the series then portal 2),

Legend of Zelda Tears of the Kingdom or Breath of the wild maybe,

Prey,

Firewatch,

Fallout: New Vegas,

FRACT OSC,

Donut County,

... This may have been more a list of my favorite games rather than play for the first time again...

Super Mario Galaxy

Final Fantasy 6

Super Mario 64

Super Mario World

Resident Evil

VVVVVV

Roller Coaster Tycoon, 2 preferably. Like, if OpenRCT2 could magically time travel back and exist on 12 year old me's computer, it would be bliss.

Factorio

Super Metroid

Minecraft, botw

Either Oblivion or Fable: The Lost Chapters

The first Mass Effect. That moment when Sovereign speaks... instant goosebumps. ("Rudimentary creatures of blood and flesh, you touch my mind, fumbling in ignorance, incapable of understanding.") This also presupposes that I haven't played the other parts as well of course. I actually played the second one before the first one back in the day.

Champions of Norrath on PS2. I played through the entire game with my dad and it was the most fun I ever him.my mother said he used to play video games all the time, but then stopped (having kids probably does this to many). So when he said he’d play the game with me i was very excited. i only wish i could have found the games sequel, return to arms, before heading off to college. that would have been a summer to remember.

i don’t know why he never shared his love of video games with either me or my sisters. some dads are weird like that. i bought him a series s for the holidays and while i don’t think he plays it, he enjoys watching his grandkids play.

CrossCode

Brothers: A Tale of Two Sons

Inside

BioShock

All great stories with some very powerful moments.

Everquest. No other game I've played before or after instilled that feeling of exploration and wonder that this game did. I'm so glad I got to experience that before I got older.

Neverwinter Nights

Picking one is quite hard, but since it's retro only, my vote goes to Super Metroid. That game is a rare gem.

Wing Commander 3 and Black & White.

Played both of them when i was a kid and it'll be nice to be able to play them again as an adult.

Stardew Valley or Terraria.