It looks like that Virtualmin uses Centos for their own Site, so the recommendation is probably CentOs?
Well, of the Grade A supported distros (listed here: http://www.virtualmin.com/os-support), I’d recommend using the one you’re the most familiar with
Those will all work well for you.
If you really don’t know which to choose – CentOS tends to get a little more testing since more folks run Virtualmin on it.
OK, I though so too, but I noticed that the latest version RHEL 6 is not supported?
Yeah, CentOS 6 (and thus, RHEL 6) will be supported a few weeks after CentOS 6 is released – which is looking like it may be in a few weeks here.