Comment by hdante

The most powerful, fully open, fully programmable desktop computer today is the Raptor Computing Systems Talos II workstation. It uses IBM OpenPOWER CPUs and the OpenBMC board firmware.

https://www.raptorcs.com/content/TL2DS1/intro.html

Openess might be impacted by add-on hardware. For servers, more powerful OpenPOWER systems are available from IBM.