Obiviously any of the 4 top riders can win this tour.
The little ITTs don't helps Froome, but if he can climb as he did in 2013 he should be more or less on the level of Contador. However, if I'm forced to decide between these two, I would rather choose Contador.
Nibali is nearly as good as these two in the mountains, but will loose some time. Thus he needs to destroy the field in the cobblestone stage again. If he can, he is for sure one of the contender for the overall win, but every race is different and I doubt that he achieve the same on the cobblestone next year.
It is difficult to compare the mountain form of Contador and Quintana. However we can assume that they are more or less on the same level. Quintana is young and can still improve, but considering the (short) ITT, the cobbles, the steep short hills, and that it is Contador we are talking about, I would choose Contador.
However if Contador really rides the Giro, and it seems so at the moment, Quintana is the favorite for me.
Btw: I do not understand, why some people think, the route would not suite Quintana. The mountains of the 2015 tour are as hard as the 2013 mountains, where Quintana was able to destroy the field and reached a comfortable 2 place behind the unbeatable Froome. Of this year are some cobbles but nearly no ITT.