Here are some frequency response graphs for the various models of iPhone.

(Note these are of a complete loop – microphone recording AND headphone jack playback –  they include the frequency rolloff of the microphone hardware.)

Flat is good – so the original iPhone wins! Then we gradually decrease frequency response as newer iPhones are released.

Look at how bad the low frequency roll-off is on the iPhone 3Gs. It’s -3dB (half the output power) at 200Hz – no wonder it sounds so thin.

The iPhone 4 response is a slight improvement over the 3GS.

The iPhone 5 frequency response is similar to the 4S.

Notice it’s the original iPhone that wins. That was about the time Apple made the best iPods too.

This graph came from http://www.faberacoustical.com