I can't speak for Cortana on desktop Windows 10 as I don't use it there, but when I had a Windows phone it was by far the best of the big three for hands-free communication while driving. It understood every command, every word, and was able to flawlessly send texts by voice and read responses back.

I have an iPhone now, but Siri is so hit-or-miss that I rarely use it for sending, and it doesn't read me received messages at all. With Windows phone when I got a text in the car I immediately heard "You got a text from __________, would you like me to read it, or ignore it?" That's what a digital assistant is made for, and it was amazing. If only Windows Phone hadn't stagnated and died, I'd happily still be on it.