Mine would be Thrive, not so much due to its current gameplay, though it is okay, but moreso due to an unwavering commitment to a vision. It has gone on for years and is essentially a scientifically close to accurate version of spore. They have almost finished the single cell stage and are working some on the transition to multicellular now as I understand it. I find it to be an impressive project as it started from the disappointment with spore and has evolved into something that I would genuinely suggest as a learning tool in middle school biology class.

Some of the assets still come from the original game, but OpenRCT2 is wonderful.

Veloren all the fucking way

Jason Rohrer’s stuff is pretty famously impressive, and a lot of it has been OS historically.

I spent a lot of time on Crawl Stone Soup, and actually got to the orb run once.

minetest is pretty cool. Highly decentralized which is nice. Very flexible, it’s just a game engine essentially.