An outsider with a decent shot might be Tony Gallopin. He almost had it in the Dauphiné. But there were not many mass sprints in that race. IIRC the Tour changed the rules a couple of years ago that favours the final winner even more in comparison to the intermediate sprints, so a green jersey winner à la Hushovd when he was with Cervélo is less likely.
Cavendish might explode in the third week and not finish the race from all his efforts at the Giro. But as we recently saw he may have sacrificed some speed for versatility. So I think it will be a battle between him, Greipel and Sagan!