The rule in this thread: One game or one series per post, and the reason why you should play it.

Let's start from this:
Quest for Glory 1-5 (GOG bundle)

QfG series is very unique CRPG, and includes very different gameplay among its each games.
QfG1 is one of the best text parser games, and its VGA remake uses point and click interface.
You can export your hero from one game and import him to the sequal.
The whole series share the same world and same characters, some NPCs will appear in different games.

References:
https://en.wikipedia.org/wiki/Quest_for_Glory
https://www.gog.com/mix/1001_games_you_must_play_before_you_die_gog_edition_1
https://www.gog.com/mix/1001_games_you_must_play_before_you_die_gog_edition
https://www.gog.com/mix/1001_video_games_you_must_play_before_you_die
https://www.gog.com/mix/1001_games_to_play_before_you_die

Tough rule. Is that also per forumgoer ?

To be safe, I'll skip some obvious classics that will necessarily be mentionned, and bring up the Men of War series. Because I only discovered them on GOG.

And they are very very special. The most "alive" RTS I ever saw, they let you play at the scale you want. You can direct whole groups at a traditional wide scale, or you can let them fight on their own and zoom into the battle, and take direct control of a small squad à la Commando, or even focus on one individual character, or randomly hop on another. There you can manage inventory, actions, search bodies, sneak around, aim at enemies either by indirect (designate target) or direct (control line of fire) commands... And zoom out and be reminded of the size of the ongoing battle, and go focus on another part of it...

It's frantic, brutal, gruesome, and very spectacularly rich in details. It's a bit of an "absolute game" in the same sense as Total War, except that it impresses through a different range of scales (back and forth from global battle to individual struggles, instead of back and forth from strategic maps to global battles). It gives the same feeling of being multiple games in one, and being the best at each component. But, in particular, being able to identify to each one of your individual soldiers transforms completely the feel of the battlefield. Each casualty could have been (or has been, or would have been) another "you", depending who you would have decided to control at which level.

So, a pretty unique game. Overlooked (by me). And yeah, has to be experienced. I would have missed out, especially as the screenshots make it look like another generic RTS.

Die by the Sword.
It would have also made for a good remake on the Wii had devs done a better job of making games for the innovative controllers that came with that console.

When it was in the catalog, I would have said Operation: Flashpoint GOTY. But it's not, so I won't.

Invisible Inc.
In this turn-based tactical stealth espionage game taking out the opposition is the last resort, and even that comes with significant consequences; that is, if you even have the means to do so. Get in, steal, and get out while drawing the least amount of attention.

It's one of the very few games which actually manages to put procedural level generation to good use. Learning the ropes on easier difficulty is a good way to go. The real meat of the game is on the higher difficulties as each escalation of the alarm/detection levels comes faster and harder. Every decision counts to get your unique agents out in one piece, while emptying safes, infiltrating data banks, and even rescuing the occasional operative. The game is only as unfair as one is inattentive or reckless.

Invisible Inc has a death spiral. Nope nope nope.