At the Tour crashes inevitably play their part and it is almost certain that at least one podium place goes to someone outside of the circle of favourites.
Out of the big four Froome has so far been the weakest mentally, but if everything goes right for him he can be an unbeatable terminator again. His below par spring campaign however rises questions whether he is able to reach the heights of 2013 or even Dauphine 2014 again. He looked like a different rider this spring, apart from one stage in Andalucia. However, he was the strongest rider in Tour 2012 after a non-existent spring campaign so one cannot draw too many conclusions from this years one.
Quintana has had the best and most consistent spring campaign out of the big four. He seemed to peak for Tirreno and was really impressive there, though less so in Pais Vasco and Romandie. I'm not sure his climbing is that much better compared to two years ago, but he has undoubtedly made a huge leap mentality-wise. He has now a self-confidence and assuredness of a champion, in a way that most reminds me of Contador. However, he looks to be much more cagey, calculating and conservative rider than Alberto. The big unknown factor is how he good/bad he is on the cobbles compared to his GC rivals. Also, the presence of Valverde in his team can be a double-edged sword.
Nibali is following his last years model of one superpeak, but the question is, is this superpeak high enough to beat his rivals when they are all there and in form. He is probably the most complete racer of the big four, and has a stage with cobbles to shine on again. However he has previously been somewhat weaker climber than other three and the fact that none of them where last year when Tour reached big mountains leaves us with a lot of questions and no answers. For me though, it very much looked like he cracked at end of the stage 8 hill climb, even if the gap was measly 3 seconds. Also if the cobbled stage is raced in dry conditions, Nibali's advantage over the others and specially Contador should be significantly lesser than in last years rain-soaked epic.
And last but not least Contador. Big question is how much his by now almost certain victory of the Giro influences his Tour form. For me it seems he is planning his main peak for the Tour as he is definetely not at his best at the Giro (at least climbing-wise, the long TT was really impressive). It is impossible that the Giro won't have any influence on his Tour form, but the thing with Contador and Grand Tours is that if none of his rivals are head and shoulders stronger than him (like Froome in 2013), then Contador wins. If he isn't crushed, he always finds the way to win. At this point it is absolutely impossible to predict if someone is able to crush him or not.
Of the other, Pinot has impressed me the most thus far. He seems to have stepped up a bit this year. How significant it is, remains to be seen. Also given the mountainious nature of this years Tour, he is likely to shine and reach the podium again, though victory is without his reach.
I won't make a specified prediction about final placings before the Dauphine and Suisse are over, but hope for an open and unpredictable Tour as there have been far too many processions in last years.