Of course IOS. It has got the best apps available.
Some apps are available on all the platform. The best one would be IOS.
There are lots of apps in IOS. So the apps that u will generally use, u will prefer they are from IOS.
Android is open for developing, it's easy n anyone can create n upload stupid apps.(Even in IOS)
Same for Windows.
But symbian is sort of dead. So nothing much to say about it.
If u want the most qualitative apps. It is IOS
That's because top developers prefer IOS. 2nd preference is android. I don't think windows come under any preference.