Mobile phones are a personal choice. Everyone has their own priorities. That's why I don't bach folk who want to use Apple / Google products.
However, my reasons for using BlackBerry are:
1 - Efficiency. Communicating is fast and takes little effort
2 - Battery life. My BB can get me through most of the day unless I really hammer it
3 - Open standards. I can use standard USB cables and copy music and videos from my computer without having to use special software. Most codecs are supported so conversion isn't required.
4 - Keyboard. BB keyboard's are the best hands down, and I love that they are making BB10 have the best virtual keyboard. So much so that I'm genuinely looking forward to owning a touch device for the first time.
4 - The community. There's a great BB community out there on Twitter and the websites. They are funny, insightful and plain stubborn just like me. It's great to be on #TeamBlackBerry.
I used to use ios and i am using blackberry os 7.1 right now. I' really like 7.1. So i've think it's the best one
As per the best OS concern i would like to say iOS is best but this is not for me because i am currently using Android based smartphone that provide the great features same as iOS, just minor difference might be shown you. Every person has different choice and different requirement so if you want to go for iOS then no problem but Android is also best for you as per your requirement.