One of the most affordable smart thermostats you can install without a C wire is the Emerson Sensi Wi-Fi Smart Thermostat. It works with Alexa and boasts DIY installation. For most applications, the C wire isn't required. You'll only need a C wire if you have a heat or cool-only system, heat pump, or you want to use Apple HomeKit.

The Google Nest Learning Thermostat (3rd generation) learns your preferred heating and cooling settings. Then it automatically creates a schedule that should maximize efficiency, so you don’t have to program the device yourself. It installs easily and supports geofencing, and it’s compatible with most home climate-control systems.