You don't talk about what you're aiming to do here. For virtualized servers, I'd definitely go with VMWare ESX (or ESXi) as it creates a standalone server for servers, plus you'd need a Windows system to run vSphere on to manage the servers.
If you're talking for home use, use the VMWare Workstation application...
But again, you need to narrow down your use case. Creating a standalone server? Play with other operating systems? What hardware do you have available on which to run this? What kind of use do you expect the VM to get? How much experience do you even HAVE with virtualization (I personally wouldn't ask a home user to just go out and play with ESXi, or any virtualization outside of a workstation context really...if they don't understand how resources work and what they're going to be doing to their home computer's RAM and processor, they probably shouldn't toy with it without reading more information ahead of time).
If you're just dipping your toes in the water try Virtualbox on your Windows/Linux system. Get the hang of virtualization and what kind of performance issues you might encounter.
Most of all...narrow down your specifications for what you're going to use it for (at work we're running an ESXi server with several Windows/Linux servers, and I virtualize a Windows system to manage it through vSphere using Virtualbox on Linux...)