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.

As a developer and a designer I want to get my hands on every platform my software may be used on. For iOS I test on iPad, iPhone 3GS, iPhone 4, and the simulator. But I only own an iPad and a 3GS, the others I borrow. I think it is also important to watch other people interact with my software using the different devices. These smart devices serve such different purposes for people that their expectations of how software should work on them varies widely.

I recommend asking around and finding some friends who own and regularly use these devices to help test your software.

If you plan on supporting any older devices in the market, you need to acquire the oldest slowest device which you plan on supporting, preferable one not upgraded beyond the oldest OS version which you plan on supporting. Old used iPod Touch's from someplace like eBay might be suitable.

It's also important to test on a newer device running the most current iOS version (and at least two test devices if you want to test with any new iOS beta versions as well as the current release.)