So far I have two games on my wishlist: Star Control 1+2 and Lands of Lore 1+2.
Which other old game not available on Steam do you feel is a must have? Only pick one title!

It has to be not available on Steam?

That kind of limits it. :P

Definitely buy Star Control 1+2 to support Toys For Bob. Then ignore SC1 (because it isn't very fun), and then ignore SC2 (although that is one of the greatest games ever made) and download The Ur-Quan Masters HD instead, which is a free open source source port of SC2 with better features, updated graphics (which are completely true to the original design), full voice acting and lots of customization options. I am not going to look up every must-have game in the GOG catalog to find out whether or not it is on Steam, sorry.

I am interested in top tier titles I haven't seen before. I didn't know Lands of Lore and Star Control existed because they are not on Steam. The same with Al-Qadim: The Genie's Curse which I got during a sale for 1 euro! I bet there are more gems like these. Thx!

Psyconauts
Tower of Guns
Tri

Pretty sure Steam has Tower of Guns.

who cares - must have games!

(they are all on Steam also... get them there as well)

In that case

Commandos BEL

Eye of the beholder series.

The Incredible Machine Mega Pack

but to stay on topic, I don't think Steam has Sid Meier's Alpha Centauri, which is also a must play game.

I think the more important question should be

Which GOG titles can run on your modern system such as Windows 8 or 10 ?

Why have a good old game that cannot run on anything after windows 2000.

The following games I bought that cannot be run on Windows 8-

Deus Ex: Invisible War

Bloodrayne

Aquanox

Interstate 82

Rune Classic

Unreal Gold

Red Faction 2

Although if you can get an old pc to run these games then I suppose I would recommend the games above to must have because I heard they are really good.

These lists may be of interest:
https://www.gog.com/mix/games_not_available_on_steam
https://www.gog.com/mix/games_on_gog_not_available_on_steam

But of the games on it the ones I can comfortably recommend are:

Dungeon Keeper 1 & 2 (they're not on Steam?!)
The Anno games
Populous The Beginning
Warhammer: Shadow of the Horned Rat (though I hear it can still be buggy and problematic)

Steam do not carry Crusader: No Remorse and No Regret?!?!?

Those shoot to the top of my list, then.

Or Master of Magic?!?!?

Add that one.

Or the Wing Commander series?!?!?!?!?!?!? No shit?!?

Defo add those.