Most popular Linux distributions Ubuntu, Redhat, Debian, Ubuntu, Mandriva, SuSe. It all depends what your do with it and how well you know it. Ubuntu seems to be the easiest to install and obtain good software for. But if you know what you are doing and need top of the line box then Redhat is still your choice.
Other will have different opinions but this is mine, just like distribution writers like to do things a bit differently then the next guy.
Dautovic 