I’ve tried my hand at Android and WP7 development, and I’ve attended a number of talks on iOS and BlackBerry development. Out of the four operating systems, WP7 is easiest for me. Some of it has to do with existing familiarity of .NET, C#, and Visual Studio, but I also think that the toolset is very well put together, and the general “how to get started” documentation is good.
Android development was not bad, but I’ve never been comfortable in the Eclipse environment and the fragmentation made it very difficult to get things done. iOS looks like it has a lot of manual labor involved, as if the tools don’t do much to help you, and to do native iOS development you need a Mac. The BlackBerry seminars I have attended made developing for it look miserable, but I have heard that the story has improved since then.
Based on my experiences and what I’ve learned and heard, of all the mobile platforms, I feel it’s easiest to develop for WP7. What do you think and why?