1. The X-Com Files
The X-Com files is a mod for the original X-Com: UFO Defense. For people who've played the Long War mods for the new X-Com games, it's very similar to that. Starting from humble beginnings control a rag-tag group of agents that die if an alien so much as breathes on them, every action counts, since even having an agent turn around to look behind them costs action points. The game also uses proper bullet trajectories, meaning that any obstacles (including your allies) need to be taken into account.
A brutally unfair game that will kill your team before they get a chance to act, sometimes literally since taking a single step will often trigger every enemy to fire at you. Knowing when to retreat becomes half the battle, and utilizing everything at your disposal is a must. And just when you think your safe, just when you think you've gamed the system, some Olympic level athlete throws a grenade halfway across the map, killing your elite squad of agents that you've spent 5 hours training up, forcing you to rethink your life choices. All of this comes together to an experience that genuinely feels like your fighting against all odds to save the world. And with so much content to boot (I have hundreds of hours, and I've probably barely touched 5% of the content within the game), this game easily earns the #1 spot for on my list.
2. Rain World
Having grown up playing flash games, each game brought a new world to explore and new rules to learn. Over time, these rules start to repeat, the tropes became stale, the genres started to blend together, and the unpredictability was lost. Rain World brought me back that sense of exploration and discovery. From A.I. that actually feels like it's hunting you, to the gorgeously stylized alien world, to Slugcat, the protagonist of the game, who's design left me devastated every time they died, Rain World made me both curious and afraid every time I discovered anything new, fully capturing the essence of trying to survive in an alien world. No game for such a long time has left me so immersed, and as such it earns the #2 spot on my list.
3. Star Control II
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4. Sword of the Stars: The Pit
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5. Minecraft
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