1- Contador- No explanation really required; goes into 2015 as a super favorite for the Tour. If he dedicates himself this offseason as he did last year, I don't see anyone able to beat him except Froome barring misfortune on the road.
2- Froome: On the same level of talent as Contador. I don't rate him as strongly in the mountains, but it is a pretty small margin between the two and he is the superior TTer on a flat course. He is the co-favorite for the Tour, and rightly so. Who can beat him realistically besides Contador (barring misfortune for either)? There likely won't be a cobble stage like the one in this Tour, and Froome's obvious weakness at this point is crashing.
3- Nibali: The guy who best placed to take advantage of the misfortune of the top 2 because he is the smartest rider of the top rated GT riders. Not on the same level as Contador or Froome in the high mountains or in the TT, but the gap isn't monstrous. I think he is the one with an outside shot of dethroning the top 2 at the tour.
4- Quintana: A lot of potential to improve between now and 2015, as age is on his side. I don't rate him as a much better climber than the Top 2. In 2013, Froome really didn't need to chase him down, and it is obvious now that Contador just had a pitiful season. He doesn't have AC's acceleration, but we will see.
5- Aru: Just going by potential here. After the Top 4, there is a huge dropoff. Valverde on palmares, but he seems to be in a slow decline. I think Purito missed his chance at a grand tour victory. Either he or Uran.