I suspect that the answers you get to this question will be largely
subjective based on different people's distro/OS preferences. That
being said, I've been very happy with RHEL7, CentOS 7, and Fedora 21
as the "controller" machine. However, I'm sure OpenSUSE, Arch, debian,
Ubuntu, FreeBSD and many others would be fine candidates as well.
I think a distro with python 2.7.x and the latest version of OpenSSH
client would be good candidates. Also, a possible indication of what
works well might be what OS/Distros that Ansible Tower is supported
on.
-AdamM