Which is the best server virtualization solution?

Purpose-built Traditional, CI, or HCI connectivity? My experience is in building Ethernet SW/HW development, test engineering networks. We use VMware, Hyper-V, and Citrix to validate on-prem and hybrid cloud. AWS, GCP, Kubernetes to validate multi-cloud interoperability involving native legacy, contemporary container, serverless, microservice workloads.
When using reference systems, VMware is my goto for replicating use-case or proof of concept networking et al. Citrix for sandboxing Hyper-V, Windows VDI, and Azure

I would like to know whether your infrastructure is converged or hyperconverged? if it is hyperconverged, I would recommend Nutanix, if it is converged VMware is your best bet. If you are an EMC shop then VXrail is your best bet. So it depends on what you want. And if you are using the Cloud, you can use AWS services for the virtual host if you are hosting it on the cloud.

1. VMware is the more popular server virtualization software and has been the pioneer in server virtualization.
2. Citrix Xenserver or Now Citrix Hypervisor is another alternative. However, a major problem that I have faced with Xenserver is that very few Backup software vendors provide full support for XenServer. Due to that, we cannot take full backup of VM (Virtual Machines ) which we can do with VMware.
3. Hyper -V is surely a contender, but from a reliability point of view, you have to find out the reference cases wherein a large number of VMs have been deployed in a production environment.
4. No experience on ProxMox.

So if budget is not a constraint – and should not be from the reliability point of view---go for VMware.

You may consider Citrix Hypervisor & Acronis Backup software as a combination if that works our to be cheaper.

Thanks

What kind of workload you are running? There are multiple solutions available. If you are looking for redundancy & scalability, I would recommend to check out Nutanix.

Other solution to recommend is NUTANIX. big winner in the HCI.
Now, everything depends on what kind of workload you have, were you plan to go and How critical is the cost of your solution.
If you plan big growth go with Nutanix.
If you have big constraint on your budget go with Proxmox, it work well,
otherwise VMWare will fit many case, it is a standard of the virtualization, not as scalable and agile than Nutanix, but cheaper.

As per my recommendation I will suggest Vmware Vsphere

I recommend an XCP-NG with Xen-Orchestra:

https://xcp-ng.com/  - Linux OS Distribution

https://xen-orchestra.com/ - Web Administration

I recommend VMware, my company is a reseller of that. It is the best solution, it has the best integration with the best cloud solution like Amazon, Azure, and Google.