This page will present the best hypervisors both paid and free.
When looking for a hypervisor you are looking for scalability and reliability and those are the main factors to take into consideration. If you are running a small or medium business and have a dedicated IT guy then go for the open source free options. The functionalities are almost identical from the expensive paid versions, the only difference is that the open source versions require a bit more maintenance.
Open Source
Proxmox : LINK
SmartOS : LINK
oVirt : LINK
Xen Project : LINK
Proprietary
FREE : Microsoft Hyper-V
VMWere ESXI :
Best solution for private users and small to meduim companies
From the research i have made, Proxmox is clearly the best solution both because it integrates a lot of functionallities and because it is very efficient. Not to mention it is fully Open-Source and Free without any limitations.