I did a quick check on
http://www.metacritic.com/games/pc/alpha/
Despite the flaws inherent in rating old games on the site, it gives a good idea of the best of the best on GOG. Please keep in mind that Gabriel Knight: Sins of the Father and Heroes of Might and Magic 3 are not rated on metacritic, or they would surely be on this list.
I agree with most of it, what do you think?
Single best rated game on metacritic that is available on GOG, with a score of 94:
Unreal Tournament 2004
Best of the Best (score of 87-91)
Freespace 2
The Longest Journey
Sacrifice
Gabrial Knight: The Beast Within
Duke Nukem 3D
Fallout
Far Cry
Descent 2
Prince of Persia: Sands of Time
Psychonauts
Descent
Also excellent (score of 80-86)
Ground Control
Tropico
Il-2 Sturmovik 1946
Giants: Citizen Kabuto
Disciples 2
MDK 2
Riven
Broken Sword: Sleeping Dragon
Celtic Kings: Rage of War
Painkiller
Arcanum
Stronghold
Empire Earth
Devine Divinity
Gabriel Knight 3
Ground Control 2
Interstate 76
Capitalism 2
Chessmaster 9000
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Well, I haven't played them all, but the ones I have, I certainly agree with. Oh, I might score a few a bit higher or lower, but they certainly all belong in the top 20%.

Phhh, IL2 Sturmovik and MDK2 should be in the 90's to be honest, flippin' public -_-

Well, IL-2 at least is pretty much where it should be.
It is an amazing game, but it isn't for everyone. So websites that cater toward the "average gamer" are going to mark it down, since the average person is going to get intimidated by it.

I don't know that The Longest Journey would be included in the top if they were only considering the average gamer as an audience.
The same could be said of the critical love for Demon's Souls. That game is hard (often to the point of just bad game design) but got lots of love.
I think what tends to happen is that when the games WE love aren't reviewed well we assume it's a conspiracy and not that we have very unlikely tastes.
For example, I still don't know how Suikoden II got a lower score than Final Fantasy VIII--but it's probably just a matter of personal taste.

Well, adventure games have gotten grandfathered in some how. They tend to get the whole "It is about the plot, not the gameplay". Whereas, sims and the more esoteric strategy games tend to actually get judged on their gameplay merits only. And most people don't want to have to figure out what flaps do :p
I have no problem with this, by the way. When I want to see if I enjoy something, I go to a review source that I tend to agree with. And honestly, the mainstream review sites are supposed to agree with the mainstream gamers. The people who don't touch themselves to the thought of getting new hats for their commies in Men of War :P

Nice list.

Metacritic is useless. Mostly, I use my brain to "rate" something instead. During the years I've discovered that even a "crapware" class game can give me good fun, so.... metacritic is useless.

I like Metacritic okay, but I take it for granted that everybody doesn't share my taste. Some of you are just wrong.