It's hard to pick just one:

• Deus Ex. It's timelessly topical despite being released in 2000. It predicted the War on Terror and a massive pandemic to name a couple.

• Spiritfarer, for maximum onion chopping. Saying goodbye to Gwen really messed me up since I became very attached to her, and I can't finish the last stretch of the game because it's too emotionally taxing.

• Undertale and wholesome fan-games like Act to Flirt.

• Half-Life 2, circa 2004 when it was a leap ahead of everything else. I was unsettled by the teaser screenshots due to how real it all seemed to be during its heyday. (I did re-capture part of that feeling with M Mod and its great yet faithful modernisation effects. Plus there's some blursed mods you can combine with it such as replacing Alyx with Krystal, voiced by the original actress.)

• Duke Nukem 3D: Alien Armageddon. It blew me away how much custom content and passion has been invested, so good that it almost felt like I was playing Duke Nukem for the first time all over again.