Aru is a huge talent and will win the Giro and Vuelta several times in his career. WIth Tour it's debatable imo. He needs a route that suits him, with no more than 40-50 km of TT. Plus he needs to outclimb Quintana, which may be an impossible thing to do.
Bardet will never win anything imo. He is way to fragile as a time-trialist and not strong enough to drop everybody in the mountains.
Barguil-see Bardet.
Betancur (when he's at his best) may be able to win the Vuelta. The short uphill finishes suit him and he can limit the damages in the Vuelta ITT, which is _/\__. He may also win the Giro if the route suits him and Quintana doesn't show up. In the Tour he'll only get a podium in the best case.
Formolo may be a bet, but he is too young and hasn't taken part in a GT yet.
Kelderman will focus on the Tour for many years starting with 2015. He will never win the Giro and Vuelta, even if he decides to focus on one of them, imo. In the Tour he'll have a chance if there will be enough km of TT. But even if this happens, I don't consider him strong enough as a climber to limit the damages to Quintana.
Kwiatkowski will never become a good climber imo. Or not good enough to challenge for the win/podium.
Majka will imo win the Giro or Vuelta, because he will not focus all hi seasons on the Tour. Plus the big names will not always start the Giro, so he has his chances.
Peraud will never be the strongest climber, nor the strongest agains the clock in any GT he'll take part in.
Pinot can easily win the Giro and Vuelta in the future, but will mainly focus on the Tour, which I don't think he'll ever win, as long as Quintana and Aru stay on their bikes or show up for the race. However, I think that he will win the Vuelta in some season, when he doesn't get the best prep for the Tour or is forced to abandon.
Rodriguez is already past it, so no.
Talansky can win a GT if there are enough kms of TT, Quintana is not there and he is better than Kelderman and Uran on the climbs. However, my bet is that he'll never win one.
Uran can win any of the GTs and will win one or even more imo. He's atm the best GC contender against the clock if we take out Froome and maybe Van Garderen. But I still think he has to improve a bit on the climbs.
Tejay will never win anything. He'll always focus on the Tour, and as we've seen, he always has a bad day. Even if he'll take lots of time on the pure climbers, he'll somehow screw it up in the mountains.
Zubeldia to take the shortcut and win the Tour without being seen
So all in all, my bets are Aru, Betancur, Majka, Pinot and Uran. But Betancur, Majka and Pinot with some ifs.