I think it's about 50-50 whether it will won from a bunch sprint or a late break (history says a largish bunch but 3 of the last 4 have come from a small group, so I dunno).
However, if it is a bunch then to my mind Kristoff deserves to be about an even money favourite. I've been backing him in bits and pieces for the last couple of months (initially when he was at 10-1, then 8-1, and now 5-1) on the basis of his phenomenal record in bunch sprints at the end of long stages. If he wins it will be clearly the biggest win of my life.
Arbitrarily using races over 240km as the benchmark (filtering out Paris Roubaix and Flanders as they are such different animals, even though he has ridden well in them too) Kristoff basically doesn't get out-sprinted:
2014
1st Milan San Remo
1st Vattenfall
8th World Champs (but 1st in the bunch sprint)
In 2013 Milan San Remo he also won the bunch sprint, albeit on for 8th place.
Sure it's a small sample size but his form this year is even stronger so if it's a bunch sprint he is the clearly the top chance. Rain wouldn't hurt either.