This week's book giveaway is in the OCAJP 8 forum. We're giving away four copies of OCA Java SE 8 Programmer I Study Guide and have Edward Finegan & Robert Liguori on-line! See this thread for details.
The "correct" platform is/are the ones that you can make money from.
You said "number of phones" in a way that indicates you're thinking of how many different brands/models of phones, but that's not the same as the number of phones of one brand. There's an awful lot of iPhones, for example, but there are a lot of different phones that run Android. Symbian currently has a big base, although my own opinion is that Symbian's day has passed.
Your best bet is to come up with an application and see what platforms will have the most demand for it. Or, you can research the platforms and see what people are complaining about and attempt to solve their complaints.
There is no commonality between the various phone platforms, not in the OS, not in the GUI, and not even much in the programming languages supported. Even the two "java" platforms of JME and Android have relatively little in common (I have to use quotes thanks to Oracle's lawyers).
Because of that, the one definite thing I can say is that it will pay to keep your business logic and UI logic rigorously separate if you want flexibility.
An IDE is no substitute for an Intelligent Developer.