Siri vs Cortana vs Google Now vs Amazon Echo: Which is the best voice control tech?

Which is the best iPhone voice control technology? And which is the best Mac voice control? Which mobile platform - or speaker setup - offers the best voice control technology: Siri, Cortana, Google Now or Amazon Echo's Alexa? Siri has made a lot of progress in the last couple of years, developing into an impressive digital assistant that can handle all sorts of tasks on your iOS devices, on the latest Apple TV and - once macOS Sierra launches in the autumn - on Mac as well. This last step is long overdue: Siri's absence on the Mac has been a glaring omission for years, especially as Microsoft has had its own Cortana voice-tech running on Windows PCs since the launch of Windows 10 last year.