IMHO MediaWiki is the best software for wikis, is the one more documented, more used and with more extensions; but in the end is a matter of what do you want to do and how much magic you want to have in your wiki :D
I also would recommend MediaWiki for these reasons.
The only drawback is the lack of a fine grained authorization mechanism: you cannot give write access to some pages and not others, or if you try there are flaws to be aware of (cf. [1])
What I tend to do for "sensitive" data is to create one MediaWiki database per private project (reusing the same MediaWiki code, see [2]).
But if you want to have only one set of authorized users, all with write access, MediaWiki is a very good choice.
[1] http://www.mediawiki.org/wiki/Security_issues_with_authorization_extensions [2] http://booleandreams.wordpress.com/2007/06/12/running-multiple-instance-of-m...