For me it was encapsulated by doom 3 for the most part. Games with strong dynamic shadows like splinter cell chaos theory and stalker with a mix of shiny specular effects is timeless for me. When I go back to these games the shadows still feel as good as they did when they came out and the lighting perfectly crafting the mood. A lot of modern games now even though their fidelity on paper is much better just always feel extremely flat to me and don't pop out of the screen like they used to. It seems like they're missing contrast and UE5 seems to make this even worse where all the games using it just look like mush
It's really hard to beat Sixth Gen(Gamecube, Dreamcast, PS2, the other one) for me.
Kind of a broad net to cast but I think all of my real favourite game graphics looks are encompassed here, anything outside that is more down to being particularly stylized or experimental, but when it comes to just straight up "doing the best that you can" style graphics, can't beat this generation.