If you haven't tried the latest rev of Echo software, it blows Siri out of the water.

Very specifically, you can speak in a conversational tone with Echo, vs barking commands at Siri.

I don't have experience with Cortana, but the Voice Recognition capabilities in the XboxOne is garbage.

I've used Siri on the iphone and Cortana on a Nokia Lumia 939 and they seem about equal. I'm sure the echo is better due to the superior mic.