I have only installed and tested a couple of these myself, but from judging by the overall features, specifications and demo versions of the ones I evaluated, I would say ISPconfig, Centos Web Panel, and Blue Onyx are my strong favorites.
My criteria for selecting one over the other for my projects include: user GUI experience, security features, server resources overhead, developer updates, and user options.
I can say one standout that I recently discovered and I am excited to try out is Blue Onyx which which seems to have a lot of rich features comparable to cPanel. However, I wonder how long this one will remain free, since it appears to have a larger organization behind its development from my interpretation.
I think it wouldn't hurt for you to setup a few test servers and see how well any them workout in your own workflow since it would not be at all cost inhibitive to do so.
My priority list now is like this:
ISPConfig (fast, secure, opensource)
VestaCP (convenient, nice, lacks file manager)
CentminMod (performance finetuned stack)
CentOSWebPanel (red it is not secure, but nice UI)
a few more panels are mentioned here