Windows Mobile is by far the most developer friendly platform. Please note however that you need Visual Studio Professional edition to write mobile applications. Other than that, developing applications is much like developing for desktop machines. No need of signing applications, application stores or annoying security warnings (only a single warning, when you first run your application). Also, the tools are in very mature state and you can focus on your application from day one rather than fighting with platform intricacies. So, I would say that Windows Mobile is the best solution for hobby projects and for small scale applications.
For mass market applications (targeting the consumer market), I think iPhone and blackberry are more suitable. These platforms seem to have a bigger share and on top of that, owners of these phones are more likely to pay for a new application.
There is also Symbian and specifically Nokia's S60, which has the biggest share in smartphones today. However Symbian is notoriously known as developer - unfriendly and I also think that owners of Symbian phones don't know/care that they can install applications in their devices. Not to mention that applications need to be signed in order to run.
Finally there is J2ME. This is not so limiting as it used to be and it has the big advantage that it targets almost every phone available. However, it has been used until today mainly for games, because every application that uses a sensitive API needs to be signed. It would be a good solution, if you were only interested in games.