The GNU Hurd. Keep laughing, it will get there.
Edit: instead of outright promoting the GNU Hurd, I will give my reasons.
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The architecture of the Hurd is very scalable compared to other kernels. It does not need much rewriting to adapt to modern technology. Rewriting the Hurd in rust is much easier than other kernels.
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It is a GNU project so if it does succeed, we can spread the Free software philosophy to others. It also means that corpo's cannot influence it.
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It is GPLv2 or later, so that is nice to have.
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It holds all the benefits of a microkernel.
There is probably more, but they are not as important to me personally.