I didn't see it :-D so if I could have a new try...

Rocket Ranger ?

Uuuuhhhh, guessing game, fun.

Screamer 4x4 maybe?

I'd guess Disciples: Sacred Lands. Thanks for an interesting giveaway.

I think that it was Thief 2 or 3. So I guess Thief 2: The Metal Age.
+1. :)

No. A little hint at the side: If you own over 500 games on GOG, chances are you already picked up all the classics by now. ;) And the Thief series is as classic as you can get (Thief Gold has been my favourite game for almost 15 years now).

Valdis Story: Abyssal City?

Nope. Two more hours and then everybody who already guessed gets a second chance.

Is it Evil Genius?

Hello there. I know I don't have an avatar picture of my own. I don't have close to any rep as anybody else on this thread.
But I do hope I also have a chance to guess and say thank you for being generous :) Congrats on achieved so many games.

My guess for your 666 game is:
The temple of elemental evil. (I would have said Scared gold, but then I saw there are 2 people guessed it, while I see your 7 no underneath the 8 possible guesses (which included Scared gold, that's strange too when you mentioned unless the correct game has mentioned:P)

Interesting. I love guessing and lurking.

24h have passed, my second guess is TALES OF MAJ'EYAL: ASHES OF URH'ROK.

My second guess is Rocket Ranger! XD

John Romero's Daikatana

Evoland?

Good guesses! Just as wrong as the other ones, I'm afraid, but those would've been games truly worthy of being the 666th game in my collection. The game you're all looking for isn't (which makes it even more devilishly hard to guess).

(Oh, and I left a review for Daikatana over a year ago as well ;P )