I love my Armored Core and Baldur's Gate for obvious reasons, but there are some absolute gems in the FOSS scene too (And I'm not even gonna mention all the indie stuff) like Mindustry, Xonotic, Zero-K, Shattered Pixel Dungeon, Unciv just off the top of my head.

Even though alot of them are just rehash of older titles or abandoned projects but even those are improved far beyond their original iteration. What I actually find problematic with FOSS games are stuff like low player count leading to a poor multiplayer experience and most games being very old school-ish (Xonotic is literally an arena shooter made on basically the Quake engine 🤷)