All others are equally (not very) likely, in my mind, except Le Pen-Bayrou. In order of decreasing likelihood:

Bayrou vs. Royal.  Bayrou wins with the right.
Royal vs. Le Pen. Royal wins 60-40.
Sarkozy vs. Le Pen. Sarkozy wins with low participation
Sarkozy vs. Bayrou - Bayrou wins with the left.

Bayrou vs. Le Pen. Nah.

In the long run, we're all dead. John Maynard Keynes