My Opinion after 10 Years Diving in the Pacific with Multiple Trips at Malpelo, Galapagos, Cocos and Socorro the best Spot for encounter Hammerheads is Malpelo. The Reason why is that there you see them on almost any Divesite and at anytime during the Day. In Galapagos for example you can see them on many Divesites but the prime is Wolf and Darwin where you stay just a couple Days out.
The Question at what spot you have the best Conditions to get close up Pictures is more difficault to answear.
Watertemperature is a Factor, but myself i cant answear it, because I saw dozens of Hammerheads in 22 Celsius Water and also in 28 Celsius Water. But its true mostly they swim close to Thermoclines.
I agree with some Topics above that nothing is a Zoo and everything can happen bust nothing must happen in each of those Destinations.
Good Luck with it.
Excuse may bad European English