These are the FLOSS games that stand out, list your own favourites or most-play games.
--//--
I would rate all of these, as worth a try:
Shattered Pixel Dungeon
Cube2:Sauerbraten
SGT-Puzzles
Andor's Trail
AssaultCube
Minetest
Neverball/Neverputt
PowderToy
0ad
Fillets-ng
Anuto TD
Xmoto/Bloboats
Flightgear
Kobo Deluxe
Enigma (oxyd)
LiquidWar5
H-Craft Championship
Numpty Physics
Wesnoth
The Dark mod
Have completed SuperTuxKart, BlobWars 1&2, Flare, Frozen Bubble, Hex-a-Hop, Holotz's Castle, SearchAndRescue II, Alex the Alligator, Project:Starfighter, Stormbaan Coureur, Trigger, all are fun.
--==--
Can find details about most of the above games here:
https://libregamewiki.org/List_of_games
FLOSS gaming is excellent, thanks to all these devs and asset creators.
--++--
BONUS TOMT:
I'm looking for the name of a FLOSS Quake1-mod, puzzle game, that was about placing gravity points, to curve a stream of particles around the level, and eventually into the goal target. (May have used irrlicht)
If anyone knows the name of this one, please let me know, it is my white-whale of games.
Spring Engine?
It's kind of like Minetest, but an RTS. (And it predates Minetest by a significant margin.) Originally conceived to be better client for Total Annihilation, but is now a completely extensible engine for real time strategy games and several FOSS games you can plug into it are also available -- Kernel Panic being a perennial (and perhaps appropriate) favorite.
Beyond All Reason is a great Spring engine game as well.
I've heard of Zero-K previously, and the Kernel Panic mod looks unique and fun.