RetroGaming

I know this is almost a stereotypical answer, but the Witcher 3. because after that game i went and read through all the books. so if i got to re experience it would be the difference of finding siri after 100 hours of gameplay and finding siri after 5000 hours of story. "find siri' is Geralt's primary motivation throughout the books. i can only imagine how satisfying and emotional that scene would feel for the first time with the weight of the books behind it.

Silent Hill 2

I've replayed that game so many times but the first playthrough hits different

Hotel Dusk.

Morrowind but it wouldn't matter because I don't have enough time to get immersed in it anymore.

FF6, FF9 and Links Awakening

Space Invaders

Asteroids

Pitfall

Dwarf Fortress was an amazing experience the first few times when you had no idea what to expect or do. My fortress designs have become much more boring (even if efficient) since, and I'm just unable to do the sort of artistic weirdness that happened during the first games.

I think this is a tie between Bloodborne and Panzer Dragoon Saga. Absolute masterpieces, both. I replay both of them at least once a year, but it would be wonderful to go back in fresh.

The Binding of Isaac.

This is the eternal game of ever. It is balanced, fair, excellent feedback (time to win), super I'm depth unlocks, one of the best meta games. Take the time to learn the basics and this game is infinite.

For retro games my answer is probably Yoshi's Island. If you include modern games I'd likely go with Hollow Knight. If VNs count then Echo.

The end of the game blew my mind out of the water in a way no other twist has ever quite captured. It might have been the mental space I was in at the time too. Whatever the case, I will always remember the first time I played it.

I also wish I had played the first Golden Sun before the second, so if I could put a duology and that caveat in there? Definitely those two.

Star Control 2

Breath of the Wild or Tears of the Kingdom. BOTW was my first Zelda game and I honestly loved it. I would sit for hours in the living room zoned in to the game with checklist paper and notebooks next to me to keep track of what I was doing. If I could do it again I'd instead use an interactive map like I did with TOTK.

As for TOTK, the Rito dungeon was my favorite and if I could experience that adrenaline pumping battle for the first time again, I would. Tulin is a baby and I love him.

I'm gonna say Skies of Arcadia, just love that game, and it saddens me to this day that Sega have abandoned it.