I think anyone you ask will be a bit biased...
I would say Ubuntu,
but OpenSUSE, Linux Mint, and all the other ones mentioned here are equity as stable and good out of the box.
Also a thing to note is that the distribution only matters to a certain point. You'll probably either be using GNOME or KDE as a desktop interface, which is pretty much the same across all these distros.
After a while you move beyond the install and initial impressions and programs and get to the core of the distribution which (according to me) is the community.
Ubuntu, OpenSUSE, Fedora, Linux Mint, etc. all have different communities with different personalities and feels. When you install a distro, also check out the community. Visit the forums, go to the IRC channels, ask questions, you will probably find a community and distro that matches your likes, needs, and personality best.
Besides that, though, it's all LInux ;)