Honestly, I couldn't care less who won the Super Bowl.
However, to keep a link to my Cowboys allegiance, I'd least want the 49ers to win. I believe they're tied with Dallas in rings total (5) and would be tied with Dallas in Super Bowl appearances if they make it (8).
As far as underdogs, and if I had to pick a team to win it all, I'd go with Cincinnati. Its community/city deserves a winner. I'm least sympathetic to LA, the city with EVERYTHING and has far more amenities to keep its citizens busy other than football. That's partially why the Rams are visitors in their own stadium.