Sailfish
"Best" is subjective, and iOS and Android both have strengths and weaknesses. On the whole they are similar in capabilities (speaking broadly, in terms of the kinds of apps and usefulness for things like directions, browsing, media playing, etc) but Android covers much broader hardware with less restrictions (and all the pluses and minuses that come with that) while iOS is exclusively on Apple hardware.
Google has Wear and Chromecast, Apple has the Apple Watch and Apple TV. Both have burgeoning solutions in home and auto. Google has Google Now, Apple has Siri. OS choice nowadays can come down to those ancillary products and services because they work better in conjunction with other products and services within their respective ecosystems (and with Apple, almost none of their stuff works with Google hardware, save for maybe Beats music). There isn't a clear best for a lot of these things, it really can come down to design, budget, or what kind of features you need.
Some people prefer the Apple ecosystem, and others prefer Google's. It's your use case and needs which determine "best". I don't limit myself to using one. I am using Windows Phone, iOS, Android, Firefox OS, and going back a while I have used BB OS, Maemo, Symbian, and Windows Mobile. "Best" always needs to be qualified and narrowed... and even then there are areas where one could argue iOS and Android are equal. Apple and Google approach privacy and security in different ways, but both ways are valid depending on your own take on those issues.
Personally, I currently prefer Windows Phone's simplicity and elegance for phone. I prefer Android on phone when I need power user features (running root-only apps, for instance) and apps that aren't available for WP. I prefer iOS for media and gaming on tablet. I prefer Windows 10 for tablets with pen/stylus and for a real desktop environment and legacy/desktop applications.
Most people who can give you a "best one" for all of mobile aren't likely doing a lot with their mobile devices, or don't have much experience with different OS'. Their opinions are still valid, of course, but they aren't worth arguing with.
I also keep my eyes open for new mobile OS'. As established as iOS and Android are, innovation often comes from new players less burdened by legacy and more open to change and trying new ways of doing things. Nokia, Microsoft, and RIM learned this the hard way when they didn't change quickly enough to respond to Apple and the iPhone.
All of the established OS' have "borrowed" from each other in multiple ways. There is some convergence because of that, and it's made the platforms more feature-rich and complete as a result.