After reading "The Wiki Way" by Ward Cunningham in 2004 I started looking for a "good" wiki engine. After a while I decided to try PmWiki -- and never regretted that choice. I use it for a couple of websites (private and public ones).
What I like most about PmWiki is its flexibility (as a PHP novice I managed to sketch one or two extensions (aka recipes) within a couple of hours) and the outstanding support via its mailing list (one of the most delightful lists I've subscribed).