I don't trust any of them. I take everything I see on the news with a HUGE grain of salt--more like a truckload of salt. Just try watching one story on NBC, then flip to CBS for the same story. It's amazing how differently the same event is reported on a different channel.
Personally, I used to find of the big 3 networks (NBC, ABC, CBS) that NBC was the least biased with Tom Brokaw, but lately there's been gun-bashing galore on there. I think FOX (at least around here) is even more conservative, but it seems at times too one sided the other way. I like unbiased news. I think Peter Jennings was pretty darn biased toward the liberals, but nowhere near to the degree that CBS was. Lately, I'm not a good person to ask b/c I don't watch much of the major news networks.
I did, however, see a local debate on PBS yesterday that actually was pretty impartial. That's been my only experience with PBS, but they were pretty impartial yesterday.