by Howard Miller.
I'm not aware that any of the "official" documentation recommends any distro or any version thereof. We recommend the minimum versions of server software. That's really your answer.
I've got servers running 12.04LTS (mostly works fine) and development boxed running 13.10 (mostly works fine). 14.10 isn't out yet but I'm guessing it will (mostly) work fine too
They'll all work. What the best route is depends on your circumstances IMO.