I like Smartthings. Is it the best? No. Is it the most functional? No. But it supports a lot of devices and it’s really easy to use. And coupled with Alexa, there’s very little you can’t do. For more complicated things where it might seem impressive, the key is often virtual switches and sensors.
I kept mine simple and used a smartthings hub to handle my zigbee needs and make up for Alexa’s shortcomings. Those two really do complement each other will, especially when you discover the usefulness of virtually switches and sensors (particularly a virtual Alexa by, which operates as a switch but behaves like a sensor, so you can trigger Alexa to do things from outside of Alexa)