Depends on your target user. If you want something really simple for developers, https://github.com/gollum/gollum is pretty neat (is what powers the Wiki system on GitHub), while if you want something simple for people who aren't developers, something like Mediawiki would fit better. Although Mediawiki requires a database, you can use SQLite (which is basically a file on disk as a DB) for it.

Dokuwiki is also a neat old-school alternative that basically treats files on disk as articles/pages, so no (other) DB needed.