What is the best device to test an iOS application on? I just enrolled as an iOS developer. I now need a device to test my app on a real device and not solely on the simulator.

The iPod Touch seems to be the cheapest option. Will this be enough to test my apps? Or would it be better to buy an iPad, iPhone 3G[S] or iPhone 4?

What device would you get to test?
That totally depends on the apps you want to do.

Do they need GPS localization? A microphone? Front or back cameras? Hi-res camera?

Besides, you can't test an iPad app on the iPhone, so you'll need an iPad no matter what.

Edit:
According to the comment, an iPod Touch would be enough for you.