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It depends on your configuration. If you want to run a native hypervisor, then vSphere is the most feature-rich but with a bit of setup difficulty. If you want an easy setup and a straight forward operation, then clearVM.
In case you want a hosted hypervisor (running on top of clearOS), then libvirt is your choice. Kimchi UI will make VMs setup a lot easier. In case you run clearOS in gateway mode, then be aware that there are still some issues with networking that need to be fixed, as mentioned here: https://wikisuite.org/item266-ClearOS-in-Gateway-Mode-Refuses-to-Route-KVMs -