Ok, I know this is meant to push GOG games first and foremost, so here are a few:

All the Anno titles (I haven't played 1404 yet, actually, but I've heard it's the best of them all, including newer ones not available here. Of the others, if you get only one, get 1701 - if you really want old school get 1602)
Day of the Tentacle Remastered
The Elder Scrolls III: Morrowind GOTY Edition (Mods. Need I say more?)
Gothic 2 (get 1 discounted, because it hasn't aged as well, but it's still good if you get used to the controls)
Rollercoaster Tycoon 1+2
The Curse of Monkey Island™

Just to name some from my library.

But if Steam games are also allowed, definitely

Age of Empires II: Definitive Edition (All the content of the HD version, now called AoE II (2013), and more, for same price as the base game from the 2013 version, plus awesome HD graphics. Actually the only game I've paid full price for in the last 20 years, I admit)
Black Mesa (Amazing fan remake of Half-Life, be aware though that it is not a 1:1 remake, with completely redesigned Xen. Supposedly almost out of Early Access)

The games I like the most are games that provide an interesting immersive atmosphere. And I like it dark.

Planescape Torment (best isometric game in history)
Darkness Within 1+2 (if you are not into story games, skip this, these are about atmosphere and puzzles)
Call of Cthulhu 2019
SOMA (can even get for free sometimes)
Dark Souls 1+2+3
Sekiro
Metal Gear Solid 5
The Evil Within (are playing the first now, moving on to second soon)
Metal Gear Survive
Deus Ex Mankind Divided
Prey
Dragons Dogma

S.T.A.L.K.E.R. almost forgot about these games, the first in the series are the best, play with mod: Complete mod for STALKER is awesome.

Minecraft, Creeper World 3, Subnautica, Galactic Civilizations 2, Portal and Portal 2

All of those I kinda feel guilty for paying as little as I did, and Minecraft was full price.

So, just to check, you would say that this game is better than the likes of:
Civilization 2 and 3, and Alpha Centauri
Some of the Contra games (yes, they have some isometric setions)
Age of Empires and many RTSes
SimCity 2000 and 3000, and The Sims
Starcraft

and many other ganes you probably wouldn't group with Planescape: Torment?

Anyway, as for games that are worth their price, I would definitely put Elminage Gothic on the list; I think it gave me over 100 hours of play time before I stopped. (That game is actually on sale right now!)

Sticking just to games I played and only up to $5.99 for one game or $9.99 for multi-game packs, because I haven't and wouldn't go higher myself:
- King's Bounty: The Legend
- Betrayal at Krondor (the fact that it's packed with BiA is just a bonus)
- Rollercoaster Tycoon 1
- Might and Magic 6-pack (well, just played 6 frankly, and tried 1 and don't consider it or 2 playable anymore in this day and age, but 3 and WoX should make the pack worth it)
- Quest for Glory series (2 and 3 would be enough to make the pack worth it, the rest as bonuses)
- Vampire: The Masquerade - Redemption
- SimCity 2000
- Divine Divinity (despite having been free and then 90% off so many times)
- Septerra Core (probably)
- Might and Magic 7 (maybe)
- Anachronox (maybe, with a guide to spare you the pixel hunting)

So it looks like virtually any game with sufficient effort and imagination put into it would be worth it if you'd just enmesh yourself into the game. It's not about finding the right game but about approaching a game the right way. Very interesting.

I'd say any game that I've managed to spend more than 200 hours on. Some of these....

Colonization 94'
Xcom 93' (openxcom)
Fallout 2
Fallout 3
Fallout NV
Fallout IV

Xcom 2012
Diablo 2
Star Trek BOTF
Sid Meiers Alpha Centauri

Oh damn, Mount & Blade Warband. Spent 1400 hours on that one. Fallout 4 comes next at 400 hours. I think I spent a long time on FONV but didn't keep track, 200 hours documented but probably 400 hours on that too. If there was no level cap I'd spend more time on 3 and NV.

Yes, Planescape Torment is one of the best games I have ever played. That doesn't automatically mean I believe the graphics are best or mechanics or anything like that. Im simply saying that Planescape Torment is one of the best games I have played, based on the experience I got while I played it. I will however still claim that Planescape Torments story is still one of the best in any game any time.
Besides, I started my post my stating that the games I like the most provides an interesting immersive atmosphere and are dark. Which I think Planescape Torment is among the best to do.

I agree, objectivity is hardly possible here. Always a balancing act, I guess, between intrinsic qualities of a game and personal circumstances.

Spelunky, Terraria (5 times over), Forager most nintendo games.