Red Hat and Mandrake are good for newbies, but I prefer something that gets your hands a little dirtier, which is why I use Gentoo. I've been using it for four months now, and haven't looked back. The install's a pain, and programs can take a while to install (15+ hours for OpenOffice), but once it's set up, Gentoo is fast, responsive, and total customized and optimized for your system.
Gentoo