If the app’s functionality is geared towards certain target groups like healthcare, law or financial services, then Blackberry and Windows are the logical choice as they have great penetration into these markets.
If the cost of creating the app for a specific platform is an issue then the Android platform is the logical choice. It is also the growing trend to develop apps for the Android platform. There are some problems on unique implementations with ActiveSync but these are likely to be ironed out soon.
However, when it comes to the actual development process, there are some who insist Android has the best development tools, database access and multitouch capability.
While some others point to Apple’s emphasis on speed and reduced battery consumption, which means working without the bells and whistles of java or .net.
It is difficult to develop a corporate app [for iOS] that leverages external xml-based data repositories due to its lack of xml parsing. This means that it will take longer to write an app for iPhone than it will for Android or Windows. Longer programming time means more costs.
The above factors can assist in your choice of OS.