After looking into this earlier, I came to the conclusion that the best for my purposes was Wordpress. Clever use of Nginx configs let's it pretend that it is really on the Rails site, and you can use XMLRPC to transfer data if you need it.
Why? Well, basically, the WP ecosystem is ridiculously superior to that of Rails CMSes, and as much as I love writing Rails I would prefer to just drop in a prebuilt plugin rather than writing code that people don't pay money for.
I soon discovered that despite being the rails solution that best fit the project, it still lagged far behind wordpress and other similarly battle-hardened software. Most of the other options didn't fare better.
Unless you need interoperability with Ruby libraries, need direct access to ActiveRe* datasources, or you have a well-defined, tightly-limited use case, I'd recommend going with wordpress or something equally mature.
Also consider the goals of a CMS vs your own goals as a developer. A CMS needs to be flexible for non developers to handle a wide range of tasks. Your own system only needs to handle the basics but be easily extendable (something which Rails should largely bring by itself.) In other words, Rails is already a very basic CMS.
Otherwise, I can't answer the question of "what's the best CMS" in general because different systems are better for different needs. I imagine the Ruby / Rails options are the same. You would likely be better off trying each of those that catch your eye and making a decision from there.