I've seen this thread floating around the top of this forum for awhile and decided it's time to chime in with my 2 cents.
I've been using various Linux distros for well over 10 years, and in that time I've tried at least 30 of them. My opinions are as follows:
Out of the RPM based distros [Redhat/ Suse/ Mandriva (Mandrake)/ etc...] I like PCLos best. PCLos 2007 is clean, light, easy to setup and configure has a nice balance of available software titles in it's repository. On the down side, it's slow release cycle due to it's tiny completely non-commercial volunteer based development process makes setting up an up to date system a bit more time consuming; in that many megs of files need to be updated upon installation, because the installation disc hasn't been updated in nearly 2 years. However, once up and running using Apt to get current is easy, just time consuming.
I'm a KDE fan so out of the DEB based distros I like Kubantu. It's Ubantu with KDE as the default windows manager and KDE based applications are standard.
However, I'm a geek, I love controlling and tweeking every aspect of my machine and nothing comes close to Gentoo in this regard. It's far and away my favorite distro. It's a complete and total pain in the ass to setup and configure, but once you've got it the way you like, it's amazing! You can compile everything on your machine just for your specific architecture and for the apps you plan on using. No bloat, and by setting compilation flags you can even further optimize your setup. I personally have setup 5 and 6 year old machines that run better than much newer ones because of the ability to get every once of performance out of the system. Gentoo's forums are active and helpful (as a matter of fact the Gentoo forums are quite helpful for finding answers with other distros as well).
I hope this helps,
Ciao