There’s so many I’ve played but I often come back to:
Infra Arcana, Battle of Wesnoth, Cataclysm: Dark Days Ahead (and it’s variants: Bright Nights, There Is Still Hope).
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BZFlag
Cataclysm dda is my main one but I also love Wesnoth. I also can’t resist dumping a few hours into frozen bubble every few years
Openspades
xmoto, freeorion, endless sky, wyrmsun, Sgt puzzles, minetest, devilutionx, supertuxkart
OpenTTD
And FreeOrion