As most questions without context, the answer is, “it depends”.

What type of app would you be developing? What would be your orientation for the app? (gaming, productivity, business, educational)

That type of information can give us a ground to tell you what mobile OS would best fit for your goal. But for most startups and mobile oriented business, you would pretty much need an app for both OS.

I think the best OS is IOS or Blackberry OS