Confluence
Previously we used Google Docs. However, in every company I've worked at I've seen this tool fail. People start of using it but after a while people forget. The main issue I think is that docs lacks key capabilities in discoverability, search, and engagement; essentially confluence does a way better job of pulling our team back in so we "remember" to use it again. We have a senior manager who is addicted to the Q&A feature and looks at it daily to help answer questions from our team.
The main tools I've seen consistently fail in large orgs for documentation has been Google Docs, Google Sites, and Wikis. All of these fail in the "living" component of "living documentation". They tend to be places where you write and things die / don't get updated / are never seen.
Have been playing with Google Sites and the main thing I'm worried about is portability if I should choose to go with something else. It seems hard to export.
Confluence lacks categories. You are stuck with hierarchy. It's more like filesystem directory structure than tags. This gets awful once you have a lot of stuff.
Confluence does not provide direct wikitext access. You might not think you need direct access, but then one day you want to turn a bunch of data into a big table.
Confluence, obviously, doesn't scale as well as MediaWiki. Nothing does. You'd hate to find yourself needing to migrate from Confluence's closed ecosystem over to MediaWiki once you hit the limits.
Confluence lacks the familiarity of MediaWiki. Pretty much everybody has seen MediaWiki in action on Wikipedia. This is comforting. People know what to expect from MediaWiki.
Is there good hosted MediaWiki? I have too much other infrastructure to run and would rather not have to run more myself.
I can't speak as much on Wikis as a category but the ones I've seen at orgs are difficult to set up and manage. I have also found that non-technical people have more trouble using Wikis that Confluence.