Confluence or Xwiki (Score 1)

I've created sites with plone and use confluence almost daily at work. If you're after an easy to use wiki then you can't go wrong with confluence. It stable, has a lot of free add ons that really add functionality, and they release frequently with updates that really improve the product. Though the 4.0 release will give me pause due to the new unified editor.

Atlassian also seems to be a decent company (no I don't work for them). They give away licenses of confluence to open source projects, and their $10 starter packs proceeds go to charity.

If you can't or won't spend the money then I'd look at Xwiki. It's an OK substitute for confluence and open source. I use it at a site that doesn't use a wiki enough to justify the licensing costs and it works well.