It's all subjective really... I think the only way to call it a masterpiece would be if it is overwhelmingly accepted as good by fans and critics. Something like Deus Ex.

I would define a masterpiece, in term of computer games, a game that will never get old.

I do only have 1 game in Mind: Tetris

It is a really old game (1984), with great sound and people still love to play it today.

Bastion.

Great game is any game which you enjoy to play a lot.

A masterpiece is a game where you can feel it is a labour of love of its developer(s). Even if you hate the game yourself.

A violin made in a big factory by machines can sound great (I presume, I have no idea how violins are manufactured), so it is a great violin. A violin created by some late master over many years into perfection is a masterpiece.

Or something like that, I guess.

You've touched an important point here, though probably in a different way that you had in mind: there are no objective criteria whatsoever in discussions like this.

My criteria for masterpiece status:

1. Use every constraint of the given art form and genre to enhance the work without an obvious way to improve on the original design.
The hardest to qualify for, and sort of unfair to several genres. RPGs (which try to emulate a believable world under ridiculous assumptions and limited budget/time) tend to fail that one on principle. The "improve on the original design" bit means the game should NOT be able to survive a genre switch.

2. Be enjoyable.
This includes not being morally repulsive, as well as having navigable interface, palatable graphics, reasonable difficulty, etc.

3. Leave an impression and/or have replay value.
This includes just thinking about the game because it's awesome, trying new strategies, seeing other plot branches/reactions to dialogue options, replaying a game with a plot twist to see all the hints you missed, and waiting for it to be released IT'S BEEN 12 YEARS GODDAMN.

In no particular order:
- Immortal Defense (name your price, min $1.75) - tower defense
- Spider and Web (free) - text adventure

Runners-up:
- Planescape: Torment ($9.99) - RPG
As close to a masterpiece as an RPG could get, neatly bypassing several design obstacles. However, blatant trap options in a game that tries very hard to remove the need for saving and loading are unforgivable.

- Escape from St. Mary's (free)
Would have been just as awesome as a graphical adventure, thus fails the "can't survive a genre switch" criterion.

- Portal, 2007 version (probably unpurchasable)
Since steam is mandatory, achievements have to count as an inseparable part of the game to be considered for masterpiece status.

True, and further to that I only know that Mona Lisa is considered a masterpiece because I've been told that it is. Without that influence, if I were to see the painting on my own for the first time I'd probably think that someone made a nice likeness of an average girl. Ditto a whole lot of "art" that my brain is not wired to get all emotional about.

From one of my favorite movies: http://www.youtube.com/watch?v=jh23gVksliQ

Far too subjective, but for me... I look to the community and my own personal tastes to decide.

People are still making Thief and Deus Ex mods. I have over a hundred games I've never played, but those 2 are the first things I install after a format.

Are people still playing it over a decade later when technology left it in the dust?

Do I chose to play those games over more modern and recent games?

If yes/yes... its probably a "masterpiece", but I don't really have that label. I'm content to just call them great and play them often ;)

I would say the game has to be created by a master. ;o)
Civilization fits the bill, X-Com Enemy Unknown scores really high on the masterpiece scale too.
They not just offer great if not unique gameplay but are also infinitly replayable.

And Pong of course. ^^