Easy.

Disco Elysium for all categories.

Openworld\sandbox: Minecraft.

It didn't invent things, but it made the most of it's raw components like proc. generation, crafting, impacting world by just placing a block, implementing basic algorythms into the game by redstone and modblock (?). And it bssically created the indie market.

Old role-playing: BG \ NWN (IDK what defines RPGs now)

Adapted tapletop systems, created an environment for your own modules, a big inspiration in this genre.

Sports: Unreal Tournament \ Quake 3 \ Counter Strike 1.6

That's when cybersports start to be known by masses and whole games begin to be made exclusively for it. Not ideal, but highly refined competitive games that made a huge impact on how games are made.

Nothing and I mean nothing ever comes close to Return of the Obra Dinn when it comes to puzzles. All the info is provided and it's all on you to finish the game (or not). It's a really unique experience. Less action and more wracking your brains out.

For RTS, I like Supreme Commander: Forged Alliance.

So, an interesting point of detail, is a game "gooder" when it perfectly executes its formula after countless iterations, like FF6 did, or is it better when it innovates in a new way, bringing together new ideas into a magical, if occasionally rough-around-the-edges, novel new approach that others start copying, as Doom did?

Also, are we looking at them from the perspective of their time, where Pac Man was once the pinnacle of gaming itself, or from a modern, more objective perspective, where Pac Man struggles to provide the same value as BotW does almost half a century later?

Baldur’s gate 2 for me.

Witcher 3

Action platform: Portal 2. It is polished to a mirror shine. I can't think of a higher "quality" game in the genre.

Judging by how many hours I’ve logged, Civ 6

For me it's Witcher 3 (open world RPG), Life is Strange (the first one and the prequel, basically interactive story, not that much gameplay), The Walking Dead by Telltale (I like them all, but Season 1 is especially great), Heroes of Might & Magic V (turn-based strategy, really great).

Edit: As a bonus, all of them are available on GOG, if you care about game ownership.

Mazewars on Mac as an early multiplayer FPS.

Black Tiger in the arcades was an excellent fantasy platformer.

Another World on the Amiga was a fantastically immersive cinematic platformer.

Wipeout 2097 on PSX was a futuristic anti-grav racer with a 10/10 for music, SFX, gameplay, lore, theme and graphics. Just exceptional.

...

In the current day, gaming is all about FLOSS games such as Cube2:Sauerbraten(FPS), 0AD(RTS), The Dark Place(stealth Thief remke) and Shattered Pixel Dungeon(roguelike) which are free forever and community driven, providing a much richer gaming experience.