I think people are looking at this question the wrong way.

Asking which media source is least biased is the wrong question. Of course C-Span would probably take the cake for most objective and non-partisan, but as a source of information, it's of limited utility.

When considering media bias (Internet, TV, radio and print), you have to consider their personal biases and their agenda. Sometimes their agenda is simple, to make money, like the network news, sometimes it's to further an ideological point of view like Fox News and HuffPo. Then you when know what to expect, it's easy to spot where the facts end and the commentary begins. Which is another one of my pet peeves, and the hallmark of bad journalism, when reporting and commentary become almost indistinguishable and it almost reads like spin doctoring. The major news outlets (network news, CNN, AP, and the major papers) are really bad for this, especially when they try to explain complex or highly subjective issues.

Whereas I find Fox, despite their almost exhibitionistic right wing agenda, actually does decent objective reporting (and commentary that even myself as a libertarian I often find anvilicious and tedious).

One of my favorite approaches to understand media bias is to compare coverage. It's a trick my dad taught me when I was young. On the weekend, he'd take three major newspapers, and have me compare coverage of the same issue by all three. And when you see the contrast in commentary, the spin on the facts, and especially the editorial decisions (what to cover and what not to, and how many column inches they put on each story).

The secret to understanding the media is to simply not take anything they say on faith and compare coverage to get a feel for each outlet's bias and agenda, which allows you to see easier what is spin and what is important.