Which smart thermostat is the most dependable?

I can't say with certainty that one model is better than another because thermostat performance can vary depending on various HVAC systems, electrical wiring, and other factors.
During my testing, all four of the smart thermostats I mentioned above—the Nest Learning Thermostat, the Ecobee SmartThermostat with Voice Control, the Resideo Honeywell Home T9, and the Amazon Smart Thermostat—performed consistently.
That means that during the days I tested them, each smart thermostat consistently adjusted the temperature, setting, and mode as expected.Before installing a thermostat, you should always consult a licensed electrician if you have any concerns regarding the thermostat's compatibility with your HVAC system.