I thought about writing this when I saw Matt bring the Mill against libertarians, analyze the problems in Iraq (just read Matt's post, don't worry about the links), and write this piece in the American Prospect about character. But I didn't, so I'll write it now: if I had to read only one political blogger, it'd be Matthew Yglesias.
It's not surprising that I agree with him on most things. He's a solid consequentialist with many of the same empirical beliefs that I hold, and that goes a long ways in terms of getting us to the same substantive positions. When you know that someone shares the same values and policy motivations as you do, and that his occupation requires him to spend more time learning about the issues, you can almost regard his head as an extension of yours -- it'll process information the way yours would and come to the conclusions you would if you learned about the issues. We are all part of one freakish utilitarian hive mind! Bzzz! (okay, now I'm joking).
Of the left-wing bloggers, I used to read Josh Marshall more often, but he tends to get focused on a single issue for days at a time. It's great if you're interested in that issue, but not if you're not. The Pandaguys always have something cool to say, but they aren't at the top in terms of sheer knowledge. Mark Schmitt has an amazing amount of policy insight (this may have something to do with why his posts get more trackbacks then comments, usually) but he doesn't post that often. Daily Kos is chock full of stuff, but it's of varying quality. I don't read Atrios often because I think other sites have more original analysis. Kevin Drum is so widely read that if he says anything cool, someone will link to it and I'll get it that way. Oh, and Tapped is one sweet magazine.