Ubuntu is one of the best Linux distributions. It got the best Linux desktop distribution and best Linux multimedia distribution reviews by Brian Proffitt as the Linux expert from the Linux Foundation (linux.com).
Ubuntu desktop, as the best Linux desktop distribution, is the result of the Canonical team and Ubuntu community spending a lot of time and resources to bring ease-of-use tools to this distribution, particularly in the area of installing Ubuntu and installing applications within Ubuntu. Ubuntu is the easiest Linux to install and configure. It just needs a click to get running. The very easy-to-understand Ubuntu documentation and the kind support from the Ubuntu community in the Ubuntu forums would make Ubuntu very recommended for new Linux users.
Ubuntu Studio, as the best Linux multimedia distribution, is an Ubuntu desktop extended with a very complete set of tools for audio, video, and graphic production. Ubuntu Studio also has format and codec support for a huge range of multimedia formats. The applications contained in Ubuntu Studio are the same or similar to those used by major studios to create cutting-edge work, so users are getting the best apps, coupled with the strong support ethos already found in the Ubuntu community.
Ubuntu has been doubted for security issues. But still, it is much safer than other operating systems. And even BackTrack (a Linux distribution for security testing purposes) is inherited from Ubuntu. So for extensibility, I have no doubt about Ubuntu. As long as it is properly configured and tweaked, Ubuntu security could be improved and much safer.