I think the most impressive performance we have ever seen by Aru was really the Monte Grappa MTT 2014. The biggest strength Aru has is that he can suffer a lot and has an amazing finish on mtf's. In Sestriere he could hardly follow Uran, Hesjedal,... on the Finestre, but on the last kilometer he still was able to drop anyone although he was probably the worse climber. Even in the Vuelta last year he only had one stage where he could really gain time, which was the queen stage. On other days where he tried to attack "early" he was always caught again, and the only other stages where he gained time on most gc rivals were stages where he attacked on the last kilometer. This is another example for Aru dropping riders who were actually stronger than him, because in Ermita de Alba (the super steep last mtf of the Vuelta) he always could barely follow the other riders and Landa set a pace which was just not high enough to drop his own leader. But when Purito accelerated on the final kilometer Aru was suddenly strong again, only lost 2 seconds and dropped every other rider.
Said differently, Aru's climbing level last year wasnt really super strong and it didn't really get better since 2014 and considering todays performance in the Dauphine I also doubt it will this year. The problem is that this is just not the level yet to be able to ever win the tdf, and most gc rider are already at least near their top level when they turn 26 (btw, does anyone know if he can still get the white jersey, he turns 26 on stage 2)