TD.
Still a flat and relatively long ITT ahead, so he'll take at least a minute and a half from Quintana and half of that, at least, from Nibali.
There are still some hard stages ahead and TD is tired from today's effort, but so are Nibali and Quintana. Pozzo and Zakarin aren't factors, so if they want to attack let them do it, because Nibali and Quintana will have to answer.
Quintana and Nibali will also attack (though I doubt that will be tomorrow), but they won't be doing it over the next 4 stages, because well, they aren't superhuman. I guess that's one of the letdowns of having so many hard stages in a short amount of time: eventually, all the riders will feel tired, so they'll hardly attack.
TD should follow Nairo and only him.
I may be wrong, but Nibali won't do anything particularly special on stages 18 and 19.