Biggest underdog for me of the group is Mir. He just has nothing for Nog in my books. Plus, say what you will about the Herring fight, Nog isn't really susceptible to a one punch KO (unless Mir amputated one of Sokoudjou's arms during training), and Nog's ground game is miles ahead of Mir's. Factor in cardio and that match should be no contest.
The most likely to upset is a bit tougher.
-Cote has the power to pull off the upset (he won't)
-Rashad has the ability to win that fight (but I don't really see him as a huge underdog in the first place)
-Huerta or Florian could stick around long enough to make BJ's cardio a concern
-Manny could conceivably lay on Rotten for three rounds (but Rob's not gonna let that happen)
-And Fitch has the all-around game that can test GSP in every area. Plus people seem to be really overlooking Fitch in this fight, which scares me because I don't get it.
Therefore, my most likely to upset pick is unfortunately Jon Fitch.