Google Now is fantastic at pulling up facts and searching the web; Siri is great for setting reminders, calling friends and controlling your device; Hound is best for handling complicated requests, like finding a hotel; Cortana is good for handling work-based requests and Alexa is killer at buying things.
There are also niche ones which some may prefer — Cyman, Alice, Robin and Jarvis. Still you can read more about Top 10 Best Virtual Assistant Apps here https://idapgroup.com/blog/top-10-best-virtual-assistant-apps/ and make your own decision.