The one with the prettiest desktop.
Admittedly, I only see it for a few seconds before opening an application, but basically there is surprisingly little difference between the distributions. For me, they all work well, and all have the same functionality. I don't care that the graphical package manager has a different name or looks a bit different. I don't care what form the packages come in. I don't care what the file manager is called.
For a new user interested in finding the best distribution for him or herself, there are some points that I would make.
Firstly, use a major "easy to use" distribution. The major distributions are not labeled "easy to use" in competition with each other, but as a comparison with the "hard to use" (or "not quite so easy to use") stuff like Linux From Scratch, Arch etc. Similarly they all have a "great range of software" to distinguish themselves from stuff like Damn Small Linux.
Secondly, try different distributions. As said in the question, personal preference is a large part of the choice, so make sure that you try the choices available. As you try different distributions, you will become more familiar with Linux rather than, say Fedora, and with your experience you can try those "not quite so easy to use" distributions. (But don't feel like you have to try them all. And don't feel that you have to change to a schedule. Something along the lines of install a new distribution rather than install a major update.)
Thirdly, try the software that is available. You may like Banshee or Amarok or something else as your media player and you should be able to install them all in any distribution. This particularly applies to the available desktops, Gnome, KDE, XFCE, LXDE etc. At least try a couple of them.
For a new user not all that interested in trying different distributions, I would suggest just going for one of the top 5 on Distrowatch. Personally I'd suggest Opensuse, because that at least manages to have lots of desktops available and working properly.