The curse of knowing about software licenses is that I can no longer enjoy video games that have a EULA. It's not that proprietary games aren't fun, but rather because I understand how stupid the agreement that you are making with the company that owns the game is. Think about it. They have the source code, they won't share it, you're still paying them for it.
Here is a parable to explain the difference between FOSS and not-FOSS. Suppose Bob lives in a home that is worth $100,000 and Alice lives in a home that is worth $10,000,000. Based only on this information, who would you rather be? OK, now what if I told you that Bob owns his house, while Alice is only renting hers? That changes the entire equation. The latest AAA proprietary game is a $10M house that you rent, while FOSS games are a $100k house that you own.
With that understood, here is a list of all of the FOSS video games that I'd actually play. They are all great and you should go check them out.
1. OpenRA
A first-rate RTS that far surpasses the original Red Alert games by Westwood studios. Personally, I have sunk many hours into this game and I'm regularly amazed at what the developers of OpenRA have achieved.
2. Cataclysm: Dark Days Ahead
If you've ever heard of the game Project Zomboid, this game is everything that Project Zomboid wishes it was. It's like 2D Rogue-like Fallout. Hard to learn, but worth it. The breadth of content in this game is enormous.
3. Dungeon Crawl: Stone Soup
A true Rogue-like game (in the sense that it's like the game "Rogue.") I've sunk many hours into this game, and I've only beat it once. DCSS is highly unforgiving. Playing it feels like playing poker with your life savings.
4. MineTest
MineTest looks like Minecraft, but it's not just "Knock-off Minecraft." Notably, the Minetest engine is better than Minecraft in a few areas. For one, there is no "bedrock" that limits how deep you can go because the chunk system in Minetest is 3D rather than 2D.
5. OpenTTD
A straight-forward and playable game. If you are the type of person who "likes trains," this is your game.
6. FreeCiv
Feels like a hybrid of Sid Meier's Civilization 1, 2 and 3, but it's infinitely better than those games because it doesn't have numerous major bugs that cannot be fixed.
7. Not a game: Godot engine
Having tried out Godot engine before, I'm highly impressed by what it can do. It certainly brings a lot of things into the realm of possibility that weren't possible before (at least, in a free software game with a free software engine.)
Right now, I cannot name a single specific Godot engine game that I deem "worth playing." It's possible there are ones, but I'm not aware of any. I'm optimistic that this will change.