I think it makes perfect sense to go off of Oblivion. After all, you wouldn't call every modern German you saw a Nazi, would you?
1) Dark Brotherhood. It would only take a contract for the entire Mages' Guild to be wiped out. And, as Listener, I know the Archmage personally.
2) Thieves' Guild. They are the only guild in Oblivion that seem to bear any sort of integrity. The Fighters' Guild, as was said before, just goes out and roots out other organizations just to get jobs. The Mages' Guild (even if we go as far back as Morrowind) are 1/2 lunatics, 3/8 people that just want you to do their bidding, and 1/8 people that actually have some kind of virtue. And the Dark Brotherhood, quite obviously, is just a tight-knit group of murderers.
3) I accidentally voted Mages' Guild, but I would think the Fighters' Guild. The Mage's Guild is just filled with eccentrics, The Thieves' Guild doesn't even exist as far as the public is concerned, and even less is known about the Dark Brotherhood. The fighters in the Fighters' Guild seem to just turn into thugs when they don't get the work they think they deserve. At least in Leyawiin, they would probably seen as nothing more than nuisances.