According to me, the most popular Q&A site is Qries, while Linked In Answers, Yahoo Answers, Quora and Answers.com among others are renowned Q&A forums. Qries enjoys a very high level of credibility mainly because of the type of people who answer the questions on Qries. The best part of Qries is that it covers all domains and subjects and you hardly find any left behind. As a business resource Q&A forums are not all that popular and rank way down at the eighth spot but for general queries, they are very popular, especially sites like Qries, which also have a robust follow-up discussion on many of the answers.
However, when it comes to general queries, Qries has started getting popular. The site is not just well moderated but has a good system to check spam and profanities which have proven to be real threats to the survival of many websites. However, Qries is not designed to be a forum; rather it is a typical Q&A site where there are 'power users' or experts who compete to answer a question whenever one is posted. Over a period of time, they will become popular depending on the quality of their answers that are graded on the basis of the readers' response.
A forum on the other hand is a discussion board where experts share their opinion more than answer questions. Yes, technically, they answer the questions but it is more in the nature of a discussion where there are multiple participants with their own set of answers and opinion. It depends on the users seeking the answers to distinguish between the answers and opinion in Q&A forums like Yahoo Answers or Answers.com and those in a Q&A site like Qries. After all, the answer will be in the details and these could be found adequately in a Q&A site like Qries and many other but may or may not be covered in as much detail in a Q&A forum.