In general, the most recent stable software versions (of Ruby, MySQL, etc) should be the best. But they won't always be the most tested, especially not by Decko users.
If you're looking for the most proven software versions, consider our continuous integration testing at Semaphore. There we tend to use the lowest supported Ruby version (to ensure backwards compatibility), even though we tend to use more recent versions when possible on live sites. Note that, because we start tests with a clean bundle install, we are using the most current gems .
We run tests in both PostgreSQL and MySQL using the default Semaphore versions. You can review their supported stack for those database engines, ImageMagick, Node.js, etc.
Our cucumber tests their use Firefox, but at present our developers predominantly work with Chrome.