There are several games on GOG that aren't either on Steam nor Epic nor anywhere else, unless you own an old physical copy of the game.

Overall i might be inclined to say that the most influential one (not necessarily best) would be Diablo. I mean this game basically spawned a while genre of hack and slash games. Funnily enough, i have tried other hack and slash games but Diablo is the only one that has truly sucked me in, maybe because there is less "OH GOD THIS WEAPON HAS +5 DPS BUT THIS ONE HAS +1% MOVEMENT SPEED WHICH ONE SHOULD I CHOOSE???? Though Grim Dawn and Path of Exile are good as well. Real shame rhere is no Diablo 2 here.

Heroes of Might and Magic 3 is basically also an exclusive, because the Steam version is said to be bad and lacking. However i am kind of frustrated because of that game as i can't get past the trolls and troglodytes scenario.

The Settlers 2:10A is a very good game but tedious. Have yet to finish it.

Icewind Dale 2 looks exciting (based on the same engine as the best game ever, after all) but i don't know if i will be able to get past the 600x800 resolution. Have yet to finish 1, first.

What do you think?

Al-Qadim: The Genie's Curse - nice story and Arabian setting
Blade Runner - so long awaited, I loved it back then, feels a lot like the movie
Clive Barker's Undying - haven't finished yet but happy to have it here, good story and magical powers

I think HoMaM3 is also on the Ubisoft store, I don't know how that version is.
It's cool that we can have Diablo here and not on the Blizzard store.