This is tough and I think it depends on what character I'm playing. My city elf would say Vaughn in a heat beat, while my human noble would insist it's Howe. I think the two are pretty close to a toss up. Anora...I don't trust her, but I don't think she's a dirtbag. She's her father's daughter: ambitious, self-righteous, and over-confident. But you can manipulate her fairly easily and avoid her doing any serious damage to your party. Loghain also isn't as big of a dirtbag as Howe or Vaughn mostly because he seems to feel remorse at the end (assuming you recruit him instead of killing him, of course). He's just a man fighting a war that ended decades ago. He can't see the present for the past. I'm not justifying what he's done, I'm just saying he has a bit more of a reason to his dirtbaggyness than Vaughn or Howe.