I like some of the choices of the OP:
3 big names: Nibali, Valverde and Purito
3 Colombians
But 2 of the old guys: Basso and Sánchez.
However I also noticed that everyone has an issue:
Nibali, the best in this race, is not in top shape. There were news he went to Burgos already behind in the schedule, and the last Km in Lagunas de Neila was painful.
Bur as others have said: If he's around by the 3rd week, he will have a good chance.
Valverde and Purito came from Tour. Giro pre-Tour proved too hard for Evans and others. Can it be different this time? 3rd week will be really hard for them.
Betancur: He hasn't raced in a long time. He was reportedly sick to enter Burgos. I have question marks (on the sickness and his fitness)
Henao: I expected more in Poland, he was close but his ITT was not good nor did he try in other stages to take some time back (he lost a place in GC per day in the last 3 stages). Vuelta has stronger competition than Poland.
Uran: He took Poland easy... Will Sky support him in case he has better chances than Henao?
Basso: He did great in Burgos, better than Sánchez
I wil be rooting for the Colombians and expect one of them in the podium.
My guess is: Two of the big3 to fight for the GC, one Colombian in Podium, another Top-5 along Basso.