It looks like the 4S had the biggest improvement over its predecessors. Each modal before and after the 4S look like they only slightly improved upon the previous model.
I'd agree, but the 4S is probably the first one to really compete with a good point-and-shoot, and it also seems better then the 5 and/or 5S in several of the shots.
Agree, 4 was a good improvement, but 4S made it it really usable one. I've updated from 4S to 6S and this weekend had a chance to capture some scenic ocean cliffs. The difference is definitely noticeable and I've got some very nice shots, but nothing to rage about. I would have happily kept using 4S if not lack of RAM made it's use hellishly slow.
I still use a 3Gs and the camera certainly isn't horrible, but the biggest issue with it and other early iPhones - which this review doesn't cover - is that it lacks any flash. That's a bigger deal than megapixels since it rendered earlier iPhones inadequate for spontaneous photography in many of the social situations people wouldn't consider bringing a proper camera to.
The original iPhone's camera (and thus the iPhone 3G's, since the 3G changed almost nothing besides the case and the cell radio) seemed to exist mostly to tick the "has a camera" checkbox. It was pretty awful at doing much beyond existing. It didn't even have the ability to change focus!
4S was indeed a big leap. When I broke my 4S, I temporarily downgraded to a 4 but missed the faster, better camera so much that I once again upgraded to the 4S.