Update: OpenStack is the obvious pick here. Proxmox is limited to 32 nodes per cluster and doesn’t resemble cloud providers at all. Proxmox is also Affero GPLv3 (which I would throw out just for that alone). ESXi/vSphere is still leading in the enterprise, but it’s “legacy” at this point and all the momentum is behind KVM and things that support it (like OpenStack). RedHat isn’t even a consideration given their open anti-FOSS practices publicly saying in their own forums they have no plans to support their distro on Mac hardware (for example).