The best choice is the choice that is right for you. If your hosting provider only gives you a few options, make your test server the same OS. If you get lots of options to choose from, give a few a try and see what you like best. I prefer Debian, and later versions now include Java packages to make Openfire install’s a little easier. Ubuntu is based on Debian, and might be a little more user friendly. Fedora/Centos/Redhat* are also popular choices. One word of caution about installing this on a VPS server: memory is going to get a little tight if you want to run anything else on it. Not impossible to do, just keep an eye on memory consumption.