My personal ranking of iPhone designs:
1. 4/4S - I don't think any other iPhone has caused such an uproar upon its announcement since the original 4. Steve Jobs compared it to an "old school Leica", and in many ways, the 4 lived up to that description. There was nothing else like it on the market, and it's still the most solid feeling iPhone that I would have no problem sitting on (not that I'd ever put any phone in my back pocket).
2. 6 - after my initial disgust at the protruding camera and massive antenna bars, this is quickly becoming one of my favourite iPhone designs of all time. It takes the iPad mini and iPad Air design philosophy and removes the dreadful chrome bezel that gets scratched really easily and replaces it with curved glass.
3. 2G - great design for its time that blew consumers away. It was sleek, simple, understated, but with a premium feel.
4. 3G/3GS - the 3GS was actually my first iPhone, because it was the one on the market at that time. But I've never been a fan of Apple's plastic-wrapped products. But it did look really good in the OtterBox Commuter case.
5. 5/5S - definitely my least favourite iPhone design. Not only was it very similar to the 4/4S in shape, but it was a step backwards in many ways. The metal case scratched easily, and it was too light for its size. This was the first time I skipped an entire generation of iPhones.