Portal 2.

Shadow of the Colossus

None. There's no such thing as an objective master piece. Games are art which is, by definition, subjective.

Edited to add, well this was way more controversial than I thought it would be.

I think masterpieces is one word.

Factorio.

Dragon Age: Origins and Bioshock

"Masterpiece" comes from the art world, and there are absolutely works everyone seems to agree qualify, such as:

• The Mona Lisa
• David statue
• David Copperfield by Charles Dickens

It's usually the best work by an artist, or at least the one that got them their recognition, and it stands out among other works in the field.

Undertale

This was a triumph

I'm making a note here

"Huge success"

It's hard to overstate my satisfaction

The Witness

A masterpiece could just refer to a piece of art from a master. It could refer to the quality of an engineering project, or the skill involved in the work's creation. Are these not objective qualities?

I don't really think the Mona Lisa is a great image, personally (it's a boring portrait), but I can still recognize that it was masterfully done.

::: spoiler spoiler
askldjfals;jflsad;
:::

RimWorld

Portal/Portal 2

Deus Ex (the original)

Minecraft

Stardew Valley

Terraria

Mirror's Edge

Chrono Trigger

Cyberpunk 2077

Hades

Subnautica

A Short Hike

Donut County

Video:
Doom

Tetris

Chrono Trigger

Table top:
Chess

Magic: the Gathering

Everdell

Azul

I'm taking this to mean games that stand out in or define their genre, are widely considered to be excellent, are timeless, and there's very little if any fat to trim.

• Super Mario Brothers - NES
• Super Mario 64
• Dark Souls - maybe Elden Ring takes over?
• Return of the Obra Dinn
• Half Life 2 - honorable mention: Left 4 Dead 2
• Diablo 2
• Doom
• Tetris
• Chrono Trigger
• Legend of Zelda: A Link to the Past
• Portal 2
• Little Nightmares - honorable mention: INSIDE
• GTA SA
• Tony Hawk Pro Skater 2

These aren't necessarily my favorite games, but games I think are well respected. I probably missed a bunch.

Bastion, Hades, Disco Elysium, Planescape: Torment, and Tyranny (even if the last act is rushed in the last two) come to mind for me

Age of Empires 2 /w The Conquers expansion pack.

Roller Coaster Tycoon 1. (2 was weaker without OpenRCT2, the real masterpiece, but idk if unfinished projects should count or not)

Quake 3 Arena, Unreal Tournament 1999 GOTY, Worms Armageddon, SimCity 3000 Unlimited, Forza Horizon 2 / Motorsport 3, Need for Speed Underground 1, Clonk! Rage, Metal Gear Solid 1/2/3, Ace Combat 4, Okami, Tokyo Jungle, Zelda BOTW, Mario Odyssey, Sven Co-Op, Killing Floor 1, Final Fantasy 7, LISA: The Painful, Everhood 1, Deus Ex 1, Left 4 Dead 1/2, Portal 2, Battlefield Bad Company 2... Champions of Norrath and Return to Arms, Diablo 1, Baldur's Gate 3 makes the list...NIER both games. Planet MiniGolf.

I could go on and on.

Holy shit somehow no one has mentioned:

Nier Automata

It counts as a masterpiece because of how well it blends game design, gameplay and story. I have played very few games as thoughtful, or that weaved the gameplay together into the story it was telling in such a meaningful way. I never thought once in my life that I would think philosophically about bullet hell but somehow Nier Automata has something profound to say and even manages to say it using bullet hell as a gameplay mechanic.

On top of all this, it also has a lot to say about classical philosophers, their works, and honestly deeply subverts things they had to say. It asks tough questions about their thoughts and ideas, once again, through gameplay. Numerous characters are named for classical philosophers: Pascal, Jean-Paul, Simone, Engels, Immanuel... (Yoko Taro obviously has feelings about how Jean-Paul Sartre treated Simone de Beauvoir.)

Further, Yoko Taro is doing something that a lot of game developers fail to manage to do: He is embracing gaming as a storytelling medium and eschewing the traditional three-act arc from film. Because gaming is not film. As Marshall McLuhan posited, "the medium is the message" and unlike other developers Taro's writing is aimed at the medium he is working in instead of leaning on the ropes and tropes of other mediums. (Referring back to above, tying the gameplay into the story, focusing on the medium)

It's basically impossible to not break down into tears at the ending.

Don't write it off because of the scantily clad anime women. Stay for the depth of the human condition. It is truly a masterwork in multiple respects.