Shenmue. Dreamcast, On Launch.

This question is interesting because even if you had a game that you really really like erased from your memory and you played it again, that doesn't mean you would still love this game as much as you like today, because you would still compare this game with other games that you have already played after you played the game you forgot about. For example, Pokémon Emerald is my first and favorte Pokémon game. I don't know how much the impact of it being my first Pokémon game influenced in how much I like it. Say it I had Pokémon Emerald erased from my memory and play it now, my perception on it would be different because I would compare it with other Pokémon games that I played like Pokémon Black 2/White 2, Platinum, HearGold/SoulSilver and FireRed/LeafGreen (but I still would like Emerald).

That being said, one game that I think that I would still be amazed the same way as when I played it for the first time is Super Mario Galaxy, even comparing it with other Mario games that I played after

The first Xenoblade, Shadow of the Colossus, Breath of the Wild, NieR Automata

Probably Portal 2.

But Sonic 1 (MD), SM64, Chrono Trigger, MGS1, Halo, Breath of the Wild... could also be good candidates, and they are from all my life.
I probably forgot some... like Metroid Prime, for example.

And I don't know if I would play Ocarina of Time for the first time again. I feel... the game has aged not that well. And I love Ocarina of Time, I played it a lot, I knew every secret. But... that was NOT the question. Even when it was still a "new game"... I read some "reader's letter" (in some videogame magazine) with a heavy criticism against Ocarina being too mystified from day 1 in the media... and whoever wrote that letter, was harsh... but... he for sure played the game, because (I remember) I thought he had some valid points there (not all of them necessarily getting my support, but valid nonetheless).
Anyways, Ocarina was great in its day, no doubt... I suffered from the Ocarina-mania fever as many millions had, during some years, and I remember "the magic" in my first games. But its gameplay would need some serious improvements today, because many things that in 1998 were just little raw edges not that well polished... now would be NOT tolerable anymore (one example: the need to CONTINUOSLY pause to change the boots... that alone... would be a reason for dismiss the game not only as the greatest game, but from the top 100. In 1998 though, you simply paused and unpaused 23 times per minute in the Water Temple, with no complaints.

The original Resident Evil. It was unlike anything I'd experienced before. The sense of mystery and discovery as I worked through the game and the story gradually unfolded was incredible. I can't recall anything matching that feeling.

Shout out to Super Mario 64 (first time controlling a character in 3D with a joystick was mind blowing for me) and Super Mario Galaxy (got it for Christmas after release and was up til about 4am playing it. Someone used the word magical to describe it here and that is spot on).

Ocarina of Time.

To experience brilliance like that for the first time is priceless. I agree, if you played it when it first came out, you’d know.

Either Mass Effect trilogy or Nier Automata. Probably Nier Automata.

Yeah part of the OoT magic was getting stuck and discussing with friends how to continue. OoT was a product of its time and indeed there are better Zelda experiences now for my kids.

OoT stays in my top 30, but has been moving up in my top 100 as well.

Why not the original Portal though? To experience that fresh again would still be great. But you would have to forget about Portal 2 as well (and Prey). Portal 2 was an upgrade in every way but the mechanics were not new anymore.

Fallout 1

It is highly replayable game, but it is not about whether or not I know its main story.

I've been playing both TTRPGs and CRPGs since 80s, but it was always FRPs (as we used to call them, Fantasy Role Play) - when Fallout launched in '97 it was revelation. Not only it used system that is very different than usual D&D mechanics, but its post-nuclear setting was completely different than what I was used to (and as big fan of Mad Max II, right up my alley).

Not knowing where to go, what to do, with danger being on every corner and 150 days counter ticking down, it got me to some fantastic situations on my first playthrough that I never experienced before or after. Oh, and that Mark Morgan's soundtrack that was perfectly capturing desolation of irradiated wasteland and was putting chills in my bones for the whole time.

It is nowhere near perfect game, but it is perfect creative storm of some great designers/artists who managed to make something truly unique on what was Interplay's B-tier project, and my favourite VG experience.