Empire Earth (Sierra, 2001, RTS):
One of my first modern (at least the release time it was modern) game. It was the main rival of the Age of Empires series, offered a whole timeline from the prehistoric age to the nano age, and later with The Art of Conquest expansion to the space age.
The game still works, even on windows 10, but only if there aren't issues with directx's directdraw.
You should get the patches too, but sadly not all of the languages supported.
https://ee.heavengames.com/downloads/lister.php?category=patches

Empire Earth II (Sierra, 2005, RTS):
The sequel changed the game's logic, implemented provinces and building limits and it generated mixed feelings in the fanbase.
Still, by my opinion it is a great game and better than Age of Empires III that got released the same time.
It should work without problems even on windows 10.
You really should get the unofficial patch that is still in development:
https://www.ee2.eu/patch/
It should work with any languages, but the extra units's names will be in english.

Empire Earth III (Sierra, 2007, RTS)
The sequel that should be forgotten.
This game marks the end times of Sierra and EE. We could say, it is the 2142 of EE.
There are only 3 civilizations and there were only 3 ages at the release (later they patched to 5). The one positive stuff is the unique aura of the game, otherwise it is really bad and crappy.

MissionForce: CyberStorm (Sierra, 1996, TBS)
Let's jump back to the past, I liked this game a lot, and apparently others too, as it got around 9/10 scores.
It will run even on Windows 10 if you turned on compatibility mode (like win95) and 256 color mode.
You should install the unofficial patch, tho don't get crash due to memory issues:
https://github.com/juanitogan/rbxit/releases/download/cs1.patch.r7/CyberStorm-10-11-Win7fix.r7.exe
A detailed description is here:
https://github.com/juanitogan/rbxit/wiki/CyberStorm-1

CyberStorm 2: Corporate Wars (Sierra, 1998, TBS)
Te sequel added factions (8, instead the two in the first game, where you were able to play with your own, as well new units. It should work fine even without patches, but here is a detailed description if something wrong:
https://github.com/juanitogan/rbxit/wiki/CyberStorm-2

Battlefield 1942 (EA, 2002, FPS)
The game we liked so much, what was really Battlefield, that had a huge community and a tons of mods... but wait, there are still a relatively big community and there are still some mods that get updates, or got just 1-2 years ago!

One of the biggest mod is Forgotten Hope (FH):
https://www.moddb.com/mods/forgotten-hope/downloads
That game should even work as standalone if you get those files and you won't get any CD key issues.
The mod has a community that still alive (at least at the weekend and at the nights:
http://www.ft31.de/
There is also a realism mod, adding historical flags and swastika signs of german planes:
https://www.bf-games.net/downloads/371/fh-realism-client-20-full.html

Forgotten Hope Secret Weapons (FHSW), still developed by the japanese:
https://www.moddb.com/mods/forgotten-hope-secret-weapon/downloads

BattleGroup42
The mod that just got it's final release some years ago. Some things maybe even better than FH and basically contains all of the bigger battle of the second world war and even the wars of the chinese conflict. And the hungarian battles- a big thumbs up!
https://battlegroup42.de/downloads/

Battlefield 1918
The game that BF1 should be. It got a lot kind of tanks, armored cars, historical uniforms. And it is historical, there are'nt infinite machineguns, only the support class have it, and tanks just appear in late maps, and modded maps of the original BF.
This game still get updates!
https://www.moddb.com/mods/battlefield-1918/downloads

There are a lot other mods, but maybe these were/are the mosdt popular

As GameSpy shut down, the multiplayer servers are not available without the patch, that also will introduce HD resolutions
https://team-simple.org/forum/viewtopic.php?id=2498
There are more stuff circling, like no-cd patches, you may need, depends what BF you can get. Without working CD key, most of the servers won't let to join.

Battlefield Vietnam (EA, 2004, FPS)
The sequel never became as popular as 1942, even though there were some improvements. It almost hasn't any community mods, EA released an 1942 mod with 3 japanese maps, but 1942 has a vietnam mod, the Eve of Destruction.

Battlefield 2 (EA, 2005, FPS)
Weird name choice, right? They may wanted to forget about Vietnam( not as it was bad, just gamers didn't want to leave 1942)
Even more weird, when years later BF1 was released.
So, this game really became a worthy successor of 1942. It also got a big community that is still alive and a lot mods.

Forgotten hope has a mod here too, Forgotten Hope 2
http://forgottenhope.warumdarum.de/fh2_downloads.php?

BattleGroup has a mod too, Battlegroup Frontlines, but it rarely gets any updates, and a lot smaller than FH2.
https://www.moddb.com/mods/battlegroup-frontlines/downloads

As I mentioned at others, Gamespy, etc, so you will need a multiplayer patch here too, called BF2HUB
https://www.bf2hub.com/home/downloads.php

Battlefield 2142 (EA, 2006, FPS)
The forgotten game, that is obviously better than 2042.
The last realy Battlefield, that has the same elements as 1942.
It still got less good scores than BF2 and never had a big playerbase and a lot mods unlike it's predessor.
2142 marked the end of an era of BF as they went from the WWII to the future and improved the engine.
After that they turned towards story-driven series, copying CoD, started with Bad Company and ended with BF4 (or Battlefield Hardline).

Doom (id Software, 1993, FPS)
Doom is actually one game that you can get on Steam, but it runs with Dosbox, while there are programs where you can launch the game with a lot custom options. On older systems Doom 95 will work perfect, but for newer systems, you should get GZDoom:
https://zdoom.org/news#p73631
it is empty, you need to get the wad files (Doom, Doom II, Final Doom (Plutonia, TNT)).
You should be aware about there are more wad versions of Doom, the latest has 4 episodes, but the others are circling on the internet too, it is easy to mix them.
What you really should try is the Brutal Doom mod, a complete overhaul that turns Doom much better.
https://www.moddb.com/mods/brutal-doom/downloads/brutal-doom-v21-rc1-fixed+
An other big mod is Moonman Doom
https://www.wad-archive.com/wad/0ce38253c1f9586d7e134e6b2519026644a5828a
You can find a tons of mods, from single maps, to completepacks.