I love DWQA pro. Free makes for a slightly limited form, but the pro version has all the advanced features I could ever need, and three beautiful Q&A themes. All the other Q&A plugins look like a joke compared to this, and I have tried eight plugins. You can integrate with BuddyPress to make a social Q&A network, integrate with MYCRED to let users gain points and get on a leaderboard. You can edit the code to customize is even more, but there is a lot of options on the settings page.