If I'm sincere, and I'm being sincere, Landa was the best climber in the Giro 2015, but the strongest climber doesn't always win. Although I think if Astana had managed him better, he could have been an even greater threat to Contador than he already was (which also means potentially winning). The problem, of course, was that Aru being Italian in Italy's GT, meant that he'd of had to totally crack (and, while this almost happened, to his credit didn't, because Aru demonstrated great character in getting through those difficult moments) for Landa to have taken over the Astana leadership.
Of course sport is not to be assessed on "what ifs" or "ands or buts," however, what I don't think can be denied is that Landa always seemed to have the smoothest pedal stroke and was the most efficient power/effort wise.
But there are, naturally, possible explinations in terms of Contador's performances, which, while very good in the mountains, weren't his best.
First off, Contador was racing the Giro with a "phase two" to have to tackle: the Tour. Landa wasn't and neither was Aru, his biggest rivals. It's possible then that Contador understood that he would enter the Giro in very good form, calculating that it would be enough to win against the actual field, but not in top form. He certainly had hoped to win the Giro without having to expend as much energy as he actually did, but against that Astana and considering the weakness of Tinkoff in the mountains, it cost him what it cost. In the end he won it more with his head than with his actual condiition, while nothing Astana did to unseat him ever worked. But the great champions also demonstrate that they are indeed great, when they can win even when not in top shape.
Secondly, perhaps the long TT (so unusual for the Italian race, at least recently) was forseen by Contador as being even more key to the final outcome then the mountains would actually have been. More then than just a way to put time into the great climbers of the race, Contador may have seen the TT as the true basis of his Giro preparation, which in this way could have compensated for a possible forseen lack of explosiveness on the big climbs. In fact both Aru and Landa were destroyed by Contador in the TT (I think sometimes people forget just how good Contador can be against the clock, as he has demonstrated throughout his career, even if there obviously were some disappointing performances). I'm only wondering if Landa lost as much time as he did, 4 min., to just poor time trialing, or did Astana tell him to leave some reserves for the final week in the mountains? Either way, any hope for Landa was seriously conditioned by his TT performance. On a curious note, it always struck me that Contador repeatedly told the Italian press that "he was satisfied with his preparation" specifying though (hermetically perhaps) that he "wasn't able to prepare for the Giro any better," which presupposes that his preparation was given a quota, beyond which it was prudent not to go (obviously with the Tour in mind).
Bottom line is that the way Contador seems to have approached the Giro was to have been able to win it at 80-85% If that's the case the Tour gets very interesting indeed. If not, on the other hand, than the Tour becomes very complicated for him. On the face of it, though, there is reason to be optimistic that we will see Alberto at 100% against Froome, Quintana and Nibali, which is good for the Tour.