As predictable as this may sound, Ubuntu 8.04 remains one of the best desktop distributions for many good reasons: it works with almost any hardware you throw at it, and has tons of features for both existing Linux users and prospective converts from Windows. openSUSE fared better on desktops than notebooks, but has a lot of organizational ideas in its interface that I'd like to see expanded on in the next release.
Fedora and CentOS may have both been derived from a common source but were dissimilar enough to be considered separately: Fedora recognized a slightly broader range of hardware, and CentOS seems a bit friendlier to newcomers and includes a broader spate of preinstalled administrative apps. Finally, Mandriva and its cousins -- SimplyMEPIS and PCLinuxOS -- all get major points for ease of administration, although each one organizes things somewhat differently. What you expect to find in one, you may find in a totally different place in the other two.