Finding the Error Room

The level with the Manx Lullaby and having time be affected by your position

seared into my memory

The dream level, especially because of the music.

Finding Stone Lake

Goddamn, man, that shit hit different

The nighttime level

Finding the Crystal Cave; Doing the grind up the space shuttle to the top of the building at the edge of the map

Everything up to the first encounter with Alyx. That environmental storytelling... chef's kiss

Hokkaido, No hints, no Mini-map, No Instinct Vision

Silent Assassin Rating

The Stag Nest

The Bar of Broken Heroes

1 turn left in the Vek hive, and being convinced for 5 minutes that there is nothing I can do to keep my 2 remaining power bars and the Renfield bomb.
And then I kept checking different possible actions for 20 minutes, until I found the one.

I found the only course of action that let me win the game.

The Hard Rain campaign, because it felt surprisingly real.

The survivors need fuel and forgot their flares. There's no fuel near the coast, they go for some time until they find a gas station and grab the fuel. They return through a hurricane and have to improvise a signal.

Finding the weird guy in the woods of the Light world

Basically everything related to Romani ranch and the strange entities that come one night.

Ghosting a level with the Path of Silence and figuring out that with a perfectly timed running jump you can leap over normal guards

Mission 6 with nothing but a Tranq pistol and a grenade launcher which I knew beforehand I would need to save for the SKULLS. That bridge and the entry area to the caves were some of the tensest stealth moments I've had

Building our own houses and watching the sunset with that C418 soundtrack in Splitscreen Coop...
fuck imma cry right now

M. Vonel being there before the booth is open

First battles with Ryuji and Morgana.

It was all so unbelievably stylish and cathartic in a way I've never seen before, and the music was just kicks ass in unfathomable ways, just... HELL YEAH.

Putting the pentagram into the pad of squares.

Abrupt and scary.

Riding Rayquaza to space; Finding Looker at the Battle Resort; Mr. Bonding; the Hoopa Event; Spiritomb in the Sea Mauville

And something that I've never heard anybody else talk about: Either all or most gym leaders can be found doing something in their freetime somewhere else in the overworld.

"Are you trying to escape? Things have changed since the last time you left the building. What's going on out there will make you wish you were back in here.

I have an infinite capacity for knowledge, and even I'm not sure what's going on outside.

All I know is I'm the only thing standing between us and them. Well, I was."

The village siege; The first encounters with the Regenerators

The battle with Phalanx

Entering the village parallel to Shovel Knight, such a cool idea and execution

The old world

Sans's basement; Gerson's dialogue during a genocide run, Mettaton's neutral ending = "TRUST ME. I LOOKED."

Honestly a lot of this game, but those I feel hit me most unexpectedly.