"The Ur-Quan Masters" is literally Star Control II, one of those classic games people rave about, and it's FOSS now.

A lot of the best games are open source engine rewrites of things that need some old game's data files, like Doom (currently UZDoom), Roller Coaster Tycoon (OpenRCT2), and Master Of Orion (1oom).

A lot of the other best games are roguelikes. There's so many fucking open source roguelikes. Brogue and Dungeon Crawl: Stone Soup are good starting points.