It's impossible to really compare winners of tours against past winners if none of them were ever truly pushed. Last year Nibali could have taken a nap on the side of the road and still finished first, this year we don't know that Froome feels tested enough to really try to lay the hammer down, certainly Contador from the Vuelta could have had a faster overall time but instead he just rode Froome's back wheel until near the end when he jumped him. In a grand tour most of the time there is simply one rider who's stronger than the others, this tour it's clearly Froome. We might not see how strong right now for the same reason that no adult runs full speed when racing with a 5 year old, there's no reason too. Quintana and the other riders (who are professionals and can probably judge this better than we can) seem to be of the unanimous opinion that Froome is the strongest, by far. Perhaps even when Quintana attacks he just gets a sense that Froome is at no point in difficulty and that's what stays his hand for the most part. If his primary opponents act like it's simply a winner's procession for Froome for the rest of the race they probably base that off something, most likely how much stronger he clearly is to them.