We are currently 9 releases behind. This includes the 3.8 series, which was devoted to WordPress, Drupal and MediaWiki performance, and resulted in "a 19.4% RPS improvement for MediaWiki workloads" (link). (I don't expect to see the same impact in production, but we are still due for a nice improvement).
3.9 and 3.12 are LTS, so it makes sense to target one of those.
3.6 becomes EOL at the end of January 2016.