I would also say I'm impressed by an effect's implementation in regards to the target hardware.
For Spider-Man: Miles Morales and Ratchet and Clank: Rift Apart, besides these games looking good in general, it's downright impressive they can do RT at 60 fps while still keeping 1080p. This is AMD hardware after all, so RT performance isn't as good as Nvidia's, there is no DLSS equivalent and even FSR wasn't present on consoles yet.
FEAR 1, 2005: The level of level destruction + dynamic lights + soft shadows + the amazing A.I. will never not be impressive IMO.
Doom 3, original version, 2004: All UI and text in game isn't jpeg, it's some sort of scalable SVG texture. Basically the UI and any kind of text in Doom 3 is HD AF, as if it was released today. It's extremely impressive. Not to mention, while the shadows in the game are ABSOLUTE in a way only Doom 3 can do it, the way the shadows are perfectly pixel mapped to geometry and the way they transitions on objects that move between light and shadow IS EXTREMELY IMPRESSIVE even today. There's no Unreal Engine 3 game (not even Arkham Knight) that has shadows as impressive as Doom 3 for example.
Far Cry 2, 2008: The way fire spreads and destroys everything in its path. How it destroys trees and how tree branches and how those tree branches organically grow back.
Resident Evil 4, 2005: The fact this kind of game was possible on GameCube and even more shockingly on PlayStation 2. From visuals, to scale, to variety, to geometric complexity on characters or enemies, to the cinematic angles
Metal Gear Solid 3, 2004: Snake Eater: Kinda same principle as RE4, but applied exclusively to PS2 this time around.