I've used Siri, Cortana and "Ok Google", and all provide a similar yet different experience.
For being productive (creating tasks/reminders), Siri is by far better, since it easilly integrates with my outlook
For web search, "Ok Google" has no competition...it's sometimes the same or better then when one does a web search (Siri and Cortana don't provide the same level of accuracy IMHO). I've tried Siri and "Ok Google" with my 6 year old, and "Ok Google" has been able to "decipher" him a lot more accurately.
Where Cortana shines is that she's embedded in Windows 10... it's pretty cool when you shout accross the room for either a quick answer (weather) or launching your song from your 1 TB hard drive. In this aspect, Cortana for Home Entertainment use is pretty cool