Informit.com has compared Debian, Ubuntu, Fedora, Suse, Xandros, Linpire and Mandriva. The best one? It all depends on your requirements.
The 9-page article starts here!
So Which One Should You Pick?
After all that, I hate to say, “It all depends,” but in many ways it's true. Your needs and budget will always play primary roles in choosing a distribution. However, if you take into consideration the criteria set up at the beginning of this article, it's easier to make a choice. If you want a user-friendly Linux distro but don't want to pay for it, consider SuSE's Evaluation Edition. You can download it from http://www.novell.com/products/linuxprofessional/downloads/suse_linux/index.html. There's plenty of support via email lists and user forums. If you're more computer-savvy, want access to thousands of software packages, and plan on running a web or FTP server, consider Debian or Ubuntu. And if you'd like a Linux desktop replacement for Windows that won't have you dual-booting or keeping two computers for different tasks, Xandros Desktop OS Version 3 Business Edition is the option you should seriously consider. At $129, it barely compares to the $399.99 you'll pay for Microsoft Office Standard Edition 2003 and $299.99 for Microsoft Windows XP Professional. Any way you look at it, Linux offers many viable Windows replacement options; so make your choice and take the leap.