Great question. If he beat Fitch convincingly I think he'd be the favorite. If he loses (and I can't see how he'll win), he'd surely be the underdog.
I just want to remind everyone that although I like BJ, he only ever defended the LW title three times, against:
Sean Sherk (7-4 in UFC, if you count Dunham win)
Diego Sanchez (11-4 in UFC)
Kenny Florian (11-4 in UFC)
And he hasn't won more than three fights in a row since 2004.
But he did smash Matt Hughes when Hughes was absolutely in his prime and seemed unstoppable. Hughes underestimated him admittedly but he still got whomped.
In my opinion if you took the Matt Hughes victories away an argument could be made that Penn is the overrated. But you can't just take victories away, BJ is a natural top talent who has had highs and lows in his career.
Sean Sherks,losses are to GSP,Hughes,Penn and Edgar
Florians losses are Penn,Sherk,sanchez and Maynard
Sanchez' losses are to Koscheck,Fitch,Penn and Hathaway
GSP's title wins.
Best wins are Sanchez,Daley and Rumble
Best wins are Koscheck and Hughes
Best win is swick
Best win is Sanchez and Alves
Best wins are Hughes,Sanchez,Florian and Sherk.
You can try and criticize anyones record.