MediaWiki is the leading wiki software worldwide. It is free, and open source. MediaWiki is used by the WikiMedia Foundation in all of it's projects for shared knowledge such as Wikipedia, Wikibooks, Wictionary etc.
The software + extensions + other related services and technologies can be composed into multi-purpose and powerful systems for individual, group and enterprise use. Of course any such solution will run up against a number of 'competitors'. This page will try to define some of the leading use cases for MediaWiki as well as comparing the solutions with alternatives in the proprietary software market.
The MediaWiki v Confluence article compares one of the biggest competitors in the enterprise market.
SaaS alternatives
There are also newcomers like SaaS provider https://clickup.com that provide a comprehensive suite of tools and integrated services centered around "getting stuff done".
Or, NotionHQ which is offering integrated AI. https://www.notion.so/
AirTable
is perhaps a case-study for what is needed in terms of a complete product definition and service offering for MediaWiki/Marketing
Most organizations - especially software vendors - have some combination of tools that they need to integrate to address basic needs. A typical example is the "edX" community which uses ReadTheDocs, Confluence wiki, JIRA tickets, Discourse, and documents in GitHub.
Not Wikis
As versatile as MediaWiki software can be, it is not the right tool for all challenges. Some related endeavors such as the WMF's initiative to provide educational or training instruction at https://meta.wikimedia.org/wiki/WikiLearn use the "best of breed" free-software Learning Management System (LMS) called edX because that solution squarely fits the need.
Another example is the amazing Discourse software for online forums. There are nice integrations so that your forums and wikis can get along.