Depends on the type of question you're answering. If you're providing data to support an answer, the data and/or study itself is the best source so the reader can evaluate it himself. Government data, independent organizations, and university studies are usually the best choice.
If you're supporting a particular interpretation of the data, news analysts are a valid source, including Fox news. The reader can interpret the reliability of the source themselves. Using a pro-left or pro-right political website as a source isn't worthless, but it definitely has some big limitations in value as a source.
Independent news organizations are usually better. Fox news and perhaps MSNBC don't have a reputation for being particularly neutral in their opinions, however. O'Reilly and Olbermann wind up being two sides of the same coin - one pushing viewpoints from the right and the other pushing viewpoints from the left.
There's a reason CNN prohibits its reporters from contributing to political campaigns. It helps protect the image that CNN is a neutral and objective news source. I think their reporters probably lean a little to the left in their personal views and that seeps out a little in their news analysis, but the network at least tries to keep personal views in check. Other major news sources, such as the Washington Post, also at try to provide a neutral, objective viewpoint. All in all, I don't think their entirely successful, mainly because more liberals aspire to journalism than conservatives, but the effort is still important in giving the news organization some credibility.