It depends. Do you care most about the content, or the community?
Right, so you need to choose a license that lets the content do what you want it to. If you want the content to make you rich, a GodKingCopyright is a good choice: that way, you can sell the content for cash at some later date. Examples: none, we're all too darn altruistic!
If you want the content to be freely copied and reused around the world, choose some form of open content, probably CopyLeft. This also grants the RightToFork, so if the community goes evil, or the site runs out of cash, the content will probably survive. Avoid the FreeDocumentationLicense, as it isn't really appropriate for wikis. Instead, look at one of the Creative Commons ShareAlike licenses. Example wikis: CommunityWiki, WikiPedia
In that case, the first question to ask is, what do you think about ForkingOfOnlineCommunities? In general, you'll want to use a DefaultCopyright if you loathe forks. Of course, individual contributors can still use PermissionTagging to let things they write be more widely copied. Example wikis: MeatballWiki, WikiWiki.
If you are comfortable with the possibility of forking, choose some sort of non-copyleft, OpenContent license. There are lots of options here, but for a wiki, good choices are PrimarilyPublicDomain, or the CreativeCommons non-commercial license ("cc-nc"). Example wikis: InfoAnarchy, KmWiki.
If you care about the community, then you'll also care what future members of your community want. Think about your TargetGroup as well as yourself: a site for professional writers should probably be default copyright, while a site for Linux enthusiasts should probably be non-copyleft open content.
Why don't I need the extra protection of copyleft for a CommunityOverContent site?
Because if someone copies the content, without the community, they're left with nothing of major value. The real value is in the community, which is uncopiable. Therefore, the danger of a hostile "embrace and extend" attack is minimal. By contrast, if you value ContentOverCommunity, then copyleft is a serious option.
If it's all a bit uncertain in your mind at the moment, choose either GodKingCopyright, or PrimarilyPublicDomain. Either of these choices will let you switch the copyright license at some later point, once you're a bit clearer about what's going to be required. Don't consider this a cop-out. If you set copyright policy too quickly, you might have completely mis-estimated your future users, and it is difficult to change copyright policy once set.
There are some other switches you can pull: for example, provided you avoid copyleft, you can always go to default copyright later on. So if you value community, but don't know whether forking makes sense for your site, or what your contributors might think about the issue, start with something like "cc-nc", and switch to default copyright later, if needs be.
Attribution is tricky on wikis, because of reworking (WhatIsReworking). If you choose a license that requires attribution, make it abundantly clear that the site's users as a whole will be given attribution, rather than individual users.
You can't please everyone all of the time: See CommunityWiki:CopyrightImpossibilityTheorem.
If you want to import stuff from elsewhere, this might make you choose a bad copyright policy, just to be able to import their content. In general, this is bad: it is better to use NearLinks and FairUse. Control your copyright situation: don't let it control you.
Unless you've selected a default copyright, make sure people know about the copyright situation: don't let them find out about it after they've formed an emotional tie to the community. Not only is that dubious legally, it won't win you any friends.
I am of course biased towards the public domain as that was my first suggestion for the copyright policy here, and prior to university my preferred choice. In the intervening years, I have been bombarded with many conflicting messages about licensing, and have concluded that making no decision is often the safest choice given the law will continue to adapt. --SunirShah
I moved a discussion between Bayle, Helmut, and Stephen onto WikiCopyrightArchived for now. It's basically discussing the question of why different people like different copyright policies, and it's interesting, but not so helpful here, I think. Anyway, how did my ReplaceWithDocument go? --MartinHarper
See also CategoryOpenContent