If you truly want the best thermostat for Apple HomeKit then the answer has to be the ecobee Smart Thermostat, especially now it has tighter Apple integration through the inclusion of Siri on board. Beyond that, though, is its class leading smart learning features, advanced smart phone app, and comprehensive HVAC feature support.
If you don’t have a c-wire, you’ll be likely set thanks to the inclusion of a power extension kit right in the box, and the ability to add room sensors to provide better whole-home temperature monitoring and occupancy detection only add to the capabilities on offer.