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I've actually tried them all and after hating Apple have ended up back within the smart home Eco system.
Siri is the worst of the bunch AI wise and struggles to understand me despite the fact I speak the bloody queens.
App wise, Amazon feels like the smart home is a secondary feature to an add/sales service, Google felt like the same but with data mining, plus split Home and Voice which was a real bore. Home (apple) feels like its been designed primarily for home automation.
Homekit brings multiple vendors products into the Apple ecosystem and if you've got an iPhone already makes the most sense IMO.
Amazon doesn't play nicely with either Google or Apple smart home devices... the on-paper compatibility doesn't seem to translate to the real world.
Not that Apple or Google play nicely with Amazon too... a Nest Hello doorbell equally does not integrate with Alexa particularly well (I believe it didn't for about a year, but recently limited support may be back).
The way this is playing out means you need to pick a horse to back: