My opinion to such thing, is just like my opinion of WP in general against other OSs. When I compare, I don't look at it in terms of form and function, as those are flexible, the most important thing is concept, if you have the right concept when you are designing you will achieve great results, within your concept though. Apple, is made to make peoples life easier. android is made as an information channel to Google. Windows Phone, helps people receive information they care about quickly.
Now in terms of personal assistance, I think no one has a sold Design vs concept but WP. to me I think they are the most committed to their Idea, which is, delivering what information is important to you.
Weirdly all bloggers are saying Cortana is Siris rival, when in real life it is not (IMO), it is competing with Google Now much more, as the concept of WP is to give you Information at a Glance, hence reminding you to tell someone something as soon as you receive a phone call, or while texting him. which in functionality looks a lot like Google Now.
Siri on the other hand follows making your life easier concept, and so it kinda does your work for you, searches for you, reserves for you, buys things... you get the idea. Cortana Does that too, but to achieve the main propose of giving you the information when needed.
Google Now which in someway should organize the information for you; under the Concept umbrella of Google, and it is really powerful at doing so, with the addition of Google Now watch. but it doesn't much follow the Android Channel stream concept which is weird. (IMO)
I like the fact that Google will introduce a device that channels the Information of Space around you. the room size and looks. now that's more like following their Android needs. 
this is a tough one. but ill vote for Cortana being the best, but again, Form and Function need a little bit of time now.
I hope im making sense.
Facebook
Twitter
Instagram