His biggest fear is deflation. His second biggest fear is hyper inflation. His third biggest fear is stagflation. But the slope of the fear curve is steep - he is much more concerned about deflation than inflation. I have not read it (trying to find out where to get it), but he wrote his thesis on the depression and also a book on it.
His last fear is to go down in history as a failure.
Ben Bernanke's greatest fear is the loss of autonomy of the Federal Reserve and thus his decision making ability being constrained or dictated by congress or other people. He wishes to act as he pleases without accountability to anyone.