I'm not sure the GP would agree with your sentiments, however, NPR pretty clearly slants to the left. All their hosts lean to the left and the composition of their multi-guest panels usually include a balance of viewpoints that lean more to the left.

Still, among popular news sources on both the left and the right I think they are about as fair as they come. Certainly on NPR the hosts are not hostile towards opposing views and you can tell that they make an effort to let dissenting callers and guests be heard even though its often clear that they disagree. I also find that NPR tends to invite guests that are respected within their community instead of guests that are intended to showcase a weak or poorly supported viewpoint.