"Having used them all, I would say Google Now is the assistant that will most easily give you a robust range of predictions, assuming you’re happy to embrace the Google ecosystem.How Google Now, Siri & Cortana Predict What You Want
If you don’t use Google services, Cortana is an interesting alternative, especially now that it’s out for Android. Just don’t expect it’ll do as much as Google Now.
If you love privacy, Apple’s Siri is appealing. But the trade-off for that privacy is going to be far fewer predictions, having to teach Siri all over again if you switch devices or have to fully reset an existing one."
Monday, September 21, 2015
How Google Now, Siri & Cortana Predict What You Want (Search Engine Land)
Final paragraphs from a digital assistant review