There are no dark horse candidates with the name id of Giuliani, but I see three potential winners in Nikki Haley, Paul Ryan, or Marco Rubio. They are young, vital players on the Republican side and have the potential to draw votes.
I do not believe that unknowns can win. Republicans never pick unknowns.
Not criticizing those people, often running one year builds name recognition up for a later run. But they wouldn’t win in 2012.