Generally, the best place to get a Linux distro is from that distro's own home page.
If you need something that is like Red Hat Enterprise Linux because you want to learn how to use RHEL, CentOS
would be an excellent choice. Scientific Linux
is based on RHEL, but more oriented to desktop usage.
Right here at LQ, there is a page that links to downloads and webpages for most major distros. http://iso.linuxquestions.org/