"best" can also depend what other devices you have got. It is probably good to try to stick to one standard operating system across many of your devices
For example if you already have an Apple computer or an iPad then and Apple smartphone with iOS is probably best for you.
But if you already have an Android device (tablet for example) then an Android smartphone is probably best.
Microsoft are still trying to sort out Windows on the various devices. I would not go for a Windows tablet or smartphone at the moment, but with Windows 10 due out later this year on all their devices (PC, tablet, smartphone, Xbox) that may be a possible solution.
As has been said, Blackberry is really a rank outsider.
Also consider the Apps that are written for the various mobile OS.
No point in going for the "best" mobile OS if there are no apps.
I think iOS has the most apps, with Android coming up a close second. Few people write for Windows mobile OS at the moment, and nobody writes for Blackberry OS.