FOX is outrageously wrong in saying that some of its shows are fair and unbiased and so is MSNBC. MSNBC basically bashes FOX and FOX basically bashes everything that isn't FOX.
Don't get me wrong though, both of these networks have some very good comedy shows like The O'Reilly Factor and Countdown With Keith Olbermann.
As far as the MOST unbiased, I would say CNN, some of their top reporters are true to the model of the "true reporter." For example, Anderson Cooper always shares his analysis or his opinion on a matter without being partisan, unlike O'Reilly or Olbermann. CNN does have some bias here and there (like Campbell Brown's show), but overall its much better than MSNBC or FOX.
Honestly I don't watch CBS, and I don't know much about it, so I won't say anything.