Some of the most polished open source games are: 0 A. D., Xonotic, Beyond All Reason, Battle for Wesnoth, Mindustry, Minetest, Thrive. These are all great games open source or not. I'm sure I'm missing a few, but these stand out to me for their completeness and polish.
I wasn't expecting to find a new "polished" open source game I've never heard of from this thread, but somehow I've never heard of Thrive before. It's even got a steam release!