At the moment, Proxmox is the best because you can create LXC containers and KVM virtual servers. It support several storage solution, including LVM-thin and ZFS.
And you can setup a HA Cluster for free, when it cost at least 500$ with VMware.
VMware is only good if you have the funds to pay for it, using a viable option for big corporations as they want an enterprise product. We personally use and like XEN. Its very easy to work with and incredibly reliable, it also helps that it's one of the most supported virtualizations among control panels, addons and other tools.
VMware is the best virtualization solution in the industry but you have to spend more money for VMware. Next we recommend Xen virtualization because its use separate memory and processor for Hyper-visor/Node and best virtualization but you must need a control panel like solusvm or onapp or xencenter to manage VMs in Xen virtualazion if you are planning to sell VPS.
Proxmox is based on KVM virtualization and container-based virtualization and manages KVM virtual machines, Linux containers (LXC), storage, virtualized networks, and HA clusters. If you are looking for a Free, it would be better. you need to spend money for subscription if you need support from them and exclusive access to the enterprise repository, stable updates and enhanced security.
I think that other virtualization technologies would not even match VMware's ESXi and VMware's functionality. It is true what @VirtuBox that VMware charges for licenses, but any top quality product costs something. As far as for ProxMox, it is very incorrect to compare with to VMware, just because ProxMox is management interface for KVM and Linux Containers, it is not a virtualization technology, itself!
KVM Is our preferred virtualization option as it gives each virtualization there own resources and individuality(lol) . Also an interface is easily built is around it. Proxmox is a tool for virtualization and does only server management. Vmware is great but the cost is only justified if you have a large customer base who can afford the price. I haven't tried the others to comment.