whichever you are more comfortable administering and securing by way of examining installed packages, system behavior, open connections, users, logs etc...
If you are looking for a pc/laptop image, I'd recommend Ubuntu or Fedora, if you want a server distro I'd recommend RedHat
Red Hat, along with Suse, are the corporate standards for servers, however my personal preference is Debian - I prefer the stability to the fuzzy gui.
What your personal 'best' is, however, depends on your comfort level between those systems. If it's Ubuntu, then that's great, but I'd recommend using what ever you are most comfortable with so you're more comfortable performing the steps required of system hardening, and then the continued process of securing, updating, and monitoring.
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