I love the 4S design, but next to the 5 it looks somehow old-fashioned.
I'd say that the 5 is my favourite, closely followed by the 4/4S, then the 3GS (I'm not really a fan of the original style, although I don't deny its great build quality).
iPhone 4/4s
The iPhone 5 looks and feels like cheap plastic. For that matter, so does the iPad Mini. Even though it is aluminium, the awful anodisation makes it look tacky.
I thought it was the 4/4S until I sold my iPhone 5 to get one. Within days I was missing the chamfered bezel and the barely distinguishable screen lines on the 5.
Probably the iPhone 4 - at the time, it was the sleekest piece of tech I'd ever seen.
Then the 2G, then very closely followed by the 5. Then the horrid plastic 3G/S.