1-on-1 fight to the death. Each have all their powers from the books, including the Silmarillion.
Personally, I think Dumbledore. Even though Gandalf owned a 5 story demon using a knife, Dumbledore is still the man as proven at the battle of the ministry in the Order of the Phoenix.
On the other hand, if Gandalf dies he'll just come back with a different colored robe and more power.