Is there a reason you are limiting yourself to maintained distributions?

Regardless of the flavor, security is truly dependent upon the configuration of the system. I agree that some distributions have a leg up on others, but I don't think you can have a "one size fits all solution". Some of the best advice ever given about security was the simplest: don't run services you don't need and keep up with patches.

As always there are plenty of sites (some very good ones have been listed in this thread) to help.

Good luck on your search.