Blackberry is still good. iOS is good-- only when the Apple Store testers are watching what they're doing, as they make mistakes that have caused huge vulnerabilities in iOS user devices. Android, sadly, is number three as it's easy to defeat security and load apps from sites where applications may have been easily tampered; unless jailbroken, Apple's delivery methods are better. Any jailbroken device is a "wildcard" because protections have also been broken.
No one knows about Windows 10, and little information is available in the US about Symbian or Tizen devices because there are few of them in the US. Summary: iOS.