Unfortunately, Apple has put themselves in a really tough situation when it comes to AI/Siri. One of the major contributing factors to Google's, and to a lesser extent Amazon's, dominance in AI virtual assistants is the massive amount of data they collect on their users. While I have no doubt that the underlying technology behind Alexa and OK Google is definitely better than Siri - having a near endless pool of data to build upon really makes a big difference. Hopefully Apple will be able to course correct Siri to an extent, to at least make it competitive. However, unless Apple drastically changes their stance on privacy (which I not only doubt they'll do, I really DON'T want them to - I love Apple's stance on privacy), I just don't see them ever being able to match or surpass the abilities of OK Google and Alexa.
With all of that being said, we are seeing Apple making strides in making Siri more useful. One of the more intriguing aspects of iOS 12 is the tight integration with Workflow - specifically in how it can integrate directly with Siri. While this doesn't resolve the underlying issues with Siri, it does go a long way in adding more and useful functionality to Siri. As far as I know, and I could very well be wrong, neither OK Google nor Alexa allow the amount of find grained control of how you want your AI to work for you as Workflow does.