My wife and four year old daughter both use Linux. They don't know it's Linux, let alone what distribution it is, yet they both use it without problem, and the reason they use it without problem is that it works and it's stable and bug free. And the reason it works and it's stable and it's bug free is it's Debian they're using.
So why isn't Debian on the list?
Multimedia used to be one objection to Debian, but as I noted in a recent post, multimedia now works out of the box in Debian, except for Flash, which is a simple install.
Another reason may be non-free software. Debian has a philosophical commitment to free software, but pragmatically makes non-free software easily available to users. ('Free' here means free as in open source, with the freedom to change, edit and add to, rather than free as in not costing money.)
As I noted in another recent post, this meant that I had to enable some non-free firmware before my video and wifi cards would work in Debian. Not a very difficult task in my case, but as this quote from a post on the Debian forum points out, it can be difficult and off-putting for new users.
Newbies, in particular... who then require a proprietary graphics driver, for example, will be at a loss as to how to find it.Other distributions make the installation of non-free software easier, or come with it already installed, so there is probably a distinction here, between the best distro for a new Linux user, and the best distro for a new linux administrator.
If I were going to install a Linux distro for a novice Linux user to try out, Debian would be at the top of my list, but if I were going to recommend a distro for a novice user to try installing, it might be a distribution that makes the use of non-free software easier.
A final reason why Debian might not be on the list is community support. The Debian documentation contains excellent support advice, but naive user questions on the Debian forum are likely to receive the answer rtfm. Similar questions may receive a more sympathetic answer on the forums of linux distributions which encourage new users.
Despite these objections, I believe new Linux users should be encouraged to try Debian. The easiest way to do so is by trying a Live System, bootable from CD or USB. There is a version stable+nonfree that contains non-free software which may make enabling computer hardware easier.
We were all new to Linux and some point. Debian works and it's stable. The documentation is excellent and makes installing non-free software an achievable task even for a non-expert user.
I reckon it's one of the best distributions for a new user, if they have some help with installation.
And if the learning curve is a challenge not an obstacle, why not try installing it?
Update: ZDNet has an article on Debian Wheezy, which details some of the issues with non-free software encountered when installing Debian on various computers.