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.