MediaWiki is probably one of the most well-engineered Wiki engines and also performs well on small scales. With a huge amount of Extensions available, it's worth a try.
Only major potential caveat with Mediawiki is that it isn't great if you need good access control for pages, it's just not made for that. I believe Dokuwiki is better at that particular aspect.
It depends on whether you can map your use case on the MediaWiki approach (namespaces, user groups). There are however Extensions to provide fine tuned access.
If you look for enterprise wikis, Twiki is another good open source software with focus on features like access control. But I don't like it for its complexity.
We are currently considering DokuWiki but heard the search is pretty slow.