Short Answer: There is no best Knowledge Management software.
That deserves some more explanation.
Knowledge Management includes components of discipline, a set of rules, processes, tools, technologies and cultural aspects. As such one will have a hard time finding a software that will do Knowledge Management for you – its mostly about exchanging experiences in relation to information available.
There are a couple of good platforms though, that are flexible enough to adapt to the principles of Knowledge Management.
Lotus Notes used to be a good one. Sharepoint is a good one. Tikiwiki, Alfresco, Drupal and WordPress are all platforms that allow you to embed and link information (content), people and processes together. If you have a Knowledge Management strategy, adapt this tools to represent it.
Document and Content management, social networking, taxonomy and workflow processing are major parts of that. Mobile access a relatively new one, but with growing importance. Include a powerful search and you are off to a good start.
KM FAQ - What is the best Knowledge Management software ?