It depends on the languages you intend to use to write the application.
If it's Java, I'd go with an android system.
If you're a c# coder I'd say any windows device would be ok.. You could likely get one on EBay for dirt cheep and for the most part you can be sure your code will run on most of them (I know this is a generality), especially if you keep the code simple.
If you're a c++ developer then the same as above, any pocket PC/winmobile will do.
If you own a mac, and you've written apps in objective c then IPhone is a good choice.
In part you have to base decisions like this on what you know going in.
If you're a blank slate.... well then you've got to figure out what direction you want to take your knowledge. I've personally picked Java and Android because I've recently been convinced it's the road to the future.