I'd go with Majka if he is relatively cheap. He's probably going to win at least one stage and finish top 10 overall. Of the GC contenders, Valverde is probably the safest bet - he'll probably win a stage and will almost certainly finish top 5 overall (probably top 3). The others are a bit more unpredictable in terms of form.
I would imagine that Bouhanni should get a lot of points as well, especially if Sagan and Degenkolb are treating it primarily as a training race.