We've used OpenVZ, Xen and VMWare.

Xen and VMWare are complete virtualization machines, but suffer in performance quickly. VMWare is expensive and Xen is time consuming to work with.

OpenVZ is extemely fast but is somewhat limited by swap and kernel modules.

So it depends what you want to do. There is no one perfect solution for everyone.

For web hosting internet servers I'd recommend OpenVZ.