Factorio and hollow knight come to mind the most easily (factorio is never on sale actually). I could also mention katana zero, dead cells, dusk, inside, supraland, firewatch.

In the sense that if you're looking for a game to buy you needn't check the sales page, just get yourself one of those (and you will probably buy just one at a time as there's no pressure). Sales / discounts is a misdirection of attention and money. Better make the games worth their full price the most prominent. Any suggestions?

Ultima 7
The Witcher 3
System Shock 2
Vampire The Masquerade Bloodlines (yes, 20$ is a lot for such an old game. But it's worth it!)

I would beg to differ; that game is really not that good, particularly with combat being so terrible. I would consider one of the more overrated RPGs, along with Baldur's gate 1 and 2 and Final Fantasy 7.

Ultima 6 alone is better than Ultima 7, and Ultima 6 comes with Ultima 5 as well for the same price. (It's also notable that the Ultima games (except the standalone Ultima 4) are on sale at the moment.)

Edit: Could those who "low rated" this post please kindly explain why?

Terraria
Far Cry 2
Fallout new Vegas
Darkest Hour

A few classics come to mind:

Civilization IV
Colonization
Master of Magic
Alpha Centauri
The Secret of Monkey Island
Indiana Jones and the Fate of Atlantis
Grim Fandango
Flight of the Amazon Queen ;)

Way too many to list.

Psychonauts. Ironically also one of the most heavily discounted games.
Enclave. Same as above. Very fair price.

Probably many (even most?) of the 5.99 ones and many of the 9.99 ones.

In addition to many of the ones named, Diablo + Hellfire.

I still maintain that GOG games at large are underpriced compared to the value we get by effectively owning the offline installers when we buy and download.

Beneath a Steel Sky - would be worth money, but is entirely free!

On the selfish and especially wallet point of views, me I wouldn't say under priced, but that there sure are super great bargains to grab (and did you say sales, a good number of them?), and offline installers of course are sure great to have.

But the way I would look at that pricing of GOG, perhaps there is a good logic behind it: best to sell loads at a good reasonable price, than not that so many as a much higher price. The former, I think, is a win-win.

SteamWorld Dig 2

It's the game that I had the most fun within the last ten years. And I played a lot of games in that time period.

Grim Dawn
Stardew Valley
FTL
Slay the Spire

Eador: Genesis
Avernum: Escape from the Pit
Age of Wonders (all up to 3rd)
Heroes of Might & Magic II & IV
Warhammer 40k: Rites of War
Missionforce: Cyberstorm
Nox
Jazz Jackrabbit 2 Collection
Incubation (in Battle Isle Platinum)
Disciples 2 Gold
KKND 2
MAX (in MAX+MAX 2 pack)

Phew, probably a lot more, but I'll stop here.