Ubuntu is one of the best Linux distributions. It got the best Linux desktop distribution and best Linux multimedia distribution review by Brian Proffitt as the Linux experts from Linux Foundation (linux.com).
Ubuntu desktop, as the best Linux desktop distribution, is the result of Canonical team and Ubuntu community support for spending a lot of time and resources on 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 need click and running. The very easy to understand Ubuntu documentation and kindly support from the Ubuntu community in the Ubuntu forums would make Ubuntu is very recommended for new Linux users.
Ubuntu Studio, as the best Linux multimedia distribution, is an Ubuntu desktop which extended with a very complete set of tools for audio, video, and graphic production. Ubuntu Studio also have 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 have been doubt with the security issues. But still, it is much more safe than the other operating system. And even BackTrack (Linux distribution for security testing purpose) is inherited from Ubuntu. So for the extendable capability, i have no doubt with Ubuntu. As long as it properly configured and tweaked, Ubuntu security could be improved and much more safe.
Technically Ubuntu and Backtrack are based on the Debian distribution so their more like siblings than parent/child. Good article with irrelevant security bit at the end.