I agree on OpenVZ. You have to learn to manage it at the command line without a good control panel but you should really be learning that anyway if you are managing VPS in a production environment.
Citrix XenServer is also pretty nice and their free version is very solid and has many options with a great control panel. They even have a bunch of really cool howto videos that show you how to do everything. If you are wanting to use Xen for high availability or some of the more advanced options you have to upgrade by adding the Xen essentials paid software .
Here is the citrix page with the videos so you can see what it looks like and how easy it is to manage. You can load both Linux and Windows VPSs on the same node with Xen.