Well, I don't know the best, but here's what you can do. If you are looking for free and open source, the current CMS systems I know are the following:
Alfresco
Cyclone3
DotCMS
Fedora
GX WebManager Community Edition
Jahia
Liferay
Magnolia
Nuxeo EP
OneCMS
You can go to this article on wikipedia to see links on articles on each of them, and you can compare features on http://www.cmsmatrix.org/ [ August 07, 2008: Message edited by: Alaa Nassef ]
There's rarely a single "best" of anything. With CMS it comes down to your requirements, about which we know nothing. OpenCMS has one of the biggest -if not the biggest- name in open source Java CMS.