The original iPhone because, well, it was the first iPhone.
iPhone 3G because of the App Store, 3G, GPS and for the first time widely available in other countries.
This is my list. "1)" is most special, "6)" is least special.
1) Apple iPhone (first generation)
2) iPhone 4
3) iPhone 5
4) iPhone 3G
5) iPhone 3GS
6) iPhone 4S
1) Apple iPhone, because it was the first iPhone. It revolutionized the market. It was a gamechanger.
2) iPhone 4, because it was a really big upgrade from the iPhone 3GS. Of course the best thing was, in my opinion, the retina display. It was the first real iPhone that had basically all features that were necessary.
3) iPhone 5, because it finally makes the iPhone really mature. Not only is it much lighter and more durable (thanks to the aluminium back that won't easily break (unlike glass)), but because it's actually an heavily improved iPhone 4. It's also more future proof thanks to the bigger display, the new dock connector that will be used throughout this decade and the new A6 chip which is a real powerhouse. It's able to beat 1.5 GHz quad-core CPUs, and it's triple-core GPU even beats the quad-core GPU found in iPad 3.
4) iPhone 3G. It was not a huge upgrade compared to the original iPhone, but it did bring a new design, GPS. It was the iPhone the first generation iPhone needed to be. Like the iPad 2 needed to be the original iPad.
5) iPhone 3GS. It brought Voice Control in a wide variety of languages (unlike Siri), finally an acceptable camera, video recording, better battery life and more. It improved the iPhone 3G in every aspect. It also became much faster - which was really necessary.
6) iPhone 4S. This was, by far, the most boring upgrade. Siri is only available in some languages and that's supposed to be the unique selling point. The better camera was great, but it wasn't necessary (unlike the 3G > 3GS camera upgrade). It also didn't need to be faster, unlike the iPhone 3GS. The iPhone 4S was a decent upgrade, but nothing special and nothing necessary.