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Thats a bit like asking what's the best food, everyone will have a very diferent oppinion :)
It all boils down to what you need/want the site to be able to acheive. I suggest taking a note of all the features you need, and visiting opensourcecms.com from there you can evaluate loads of opensource cms applications without the need to install them on your website.
Then when you find the app that is best suited to your needs you can download and customise it to be an even closer match.
We have a Content Management forum [webmasterworld.com], essentially devoted to CMS systems... and Drupal, Joomla, and Wordpress are among the most commonly discussed platforms over there. Each has its strengths and weaknesses.