In the 2020s I have used BUCK, BUCK2, and Blaze every working day. I don’t feel like getting in to details but I’ll just say that BUCK2 is the best of the 3 and I’m delighted it’s open source.

I don’t have experience with Bazel as it exists externally but I expect the internal Blaze has advanced far beyond it by now as the entire Google engineering world depends on it.