DISCLAIMER: First, I want to apologize for the admittedly clickbait title, I couldn’t think of anything better given the topic. Second, the following games and links do not involve Free-to-Play mechanics, piracy, or grey market sites. Third, I am not associated with any of the following developers or (if applicable) publishers.
My reasoning for this post is because I am sick (almost literally) of what AAA gaming has become, and it is too much of a hassle to sift through the indie games market. As such, my logic is this: The only way to go is back.
1. First on my list is the [original DOOM released back in 1993-94](http://store.steampowered.com/sub/18397/).
You would be hard pressed to set up a multiplayer match by itself, but with Zandronum, you would be able to play not just in its original form, but heavily modded as well, on servers from all parts of the globe. Main benefits compared to running vanilla are mouse support, massive support for increased refresh rates (works with my 120 Hz setup) and higher resolutions (works in 4K and possibly higher with relatively low requirements).
2. Halo: Combat Evolved on PC.
This listing pertains more to its modded variant, Halo: Custom Edition. It amazes me how large the population of this game is to this day, typically peaking around 1000 players a day. Between the numerous custom maps (which sometimes include custom vehicles and weapons) and general versatility (when used with HAC2, enables automatic map downloads, FOV adjustment, and more), it makes for a great game to go for. If you just want to play the campaign, the 1.10 patch removed the CD-check and enabled support for 4K resolutions. The main drawback is that you will have to somehow obtain a physical copy of the game and (for certain members) you will have to dig up your optical drive again 
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3. [Renegade X](https://renegade-x.com/)
This is a fan remake of the multiplayer portion of Command & Conquer: Renegade. Unfortunately, it has been suffering from a low population as of recently, with only 1 server being populated at a given time. Here is hoping this thread gives it the boost it needs.
4. Recently revived fan project [Mechwarrior: Living Legends](https://mechlivinglegends.net/)
After being shut down by what was presumably a C&D by IGP (then publisher of Mechwarrior Online), it has been brought back from the dead, and is better than ever, with new features and fixes. For those who want their Mecha fix and don’t want to give IGP (or Piranha Games) any time of day.
5. Star Wars Battlefront 2 (2005)
No, not the EA crap version, the real deal B|. As of a couple weeks ago, it received an update allowing it to play multiplayer online again (since Gamespy shut down).
6. Supreme Commander: Forged Alliance Forever
Just as a note, you will require a copy of SupCom:FA in order to use this community maintained version. For those of you familiar with Total Annihilation (which I equally recommend w/ the Total Mayhem mod btw), it is a Real Time Strategy which involves massive armies (upwards of 500 units per player, 8 players max) and the chaos that comes with such large battles.
Honorable mentions of games I was unsure where to put on this list include: Age of Empires 2 (any version), Insurgency, Star Wars: X-Wing Alliance (via Game Ranger, joystick or controller required or it simply won’t start), and Heroes of Might and Magic III: Complete (found on GOG, played online via Game Ranger, best played with the HoMM 3 HD mod).
I await your feedback, and I hope to see you in one of these games.