Good question. I had to mull it over thinking about which I would give up if forced to choose.
We use the Alexa reminders, integrated grocery app skill (OurGroceries), and to-do list skill (ToDoist) the most. Combined, they are like having a personal secretary. That'd be what I'd part with last. Because of the smarthome addition, voice control is already getting moderate use and growing. It's a handy convenience, but not a must-have. <added> Louise uses VC a lot more than I do.
Next-most used and popular would be the thermostats and their app. I'd have to pry the app out of Louise's hand if we were to give those up.
That puts the smarthome stuff last, but we'd sure miss it if it had to go. A good bit of what it does is automatic and behind the scenes, so we don't think about it --like water heater timers, driveway lighting at sunset, etc. It just does its thing. But we've also become accustomed to other features such as lights on when someone enters the room, or controlling lights in distant parts of the house, etc. The security stuff -exterior lighting, vehicle detection, motion near the front entrance- make us feel like we have more awareness and control of the perimeter and are possibly better prepared.
