Need to define "favorite".
In this context, it can mean "who do you think is most likely to try", "who would you most like to try", or "who is most likely to successfully beat him in the primary".
My answer to the last two would be no one. Some oddball might take a shot, probably just to get attention or rake in campaign contributions to pay their family members for "consulting", but at this point unless he shoots someone on fourth street he's the pub nominee.