The Nest Thermostat E is Google's entry-level smart thermostat. The E has (almost) all of the same features as the Nest Learning Thermostat, but has less expensive hardware components to keep the price down.
The Honeywell Lyric Smart Thermostat can talk with Siri through HomeKit or with Alexa via the Amazon Echo, Echo Dot and Tap speakers.
It also has geofencing capabilities tied to your phone's location. When you leave home, the Lyric should automatically adjust to a more energy-efficient temperature. And when you return, it should autoadjust back to a more comfortable setting.
Johnson Controls' $319 GLAS Smart Thermostat has a built-in Microsoft Cortana speaker. Ask Cortana to "change the temperature to 68 degrees" or to "switch from home to away mode." The GLAS also works with Alexa and Google Assistant if you have a compatible smart speaker at home.
This thermostat has a see-through OLED touchscreen display and gives you air quality reports.