Sony 7506. It's industry standard.
Do not pay attention to frequency response numbers-- they, along with all of the other specs given to you on a product page are largely useless for headphones.
What statistical information is more important however, is actual measurements.
http://www.innerfidelity.com/headphone-data-sheet-downloads
http://www.headphone.com/learning-center/build-a-graph.php
A bit of an insight on how to determine what's flat in headphone fr measurement. Since headphones rest right on top of your ears, a 'flat' response headphone should have upper midrange and lower treble rolloff to accommodate for hrtf. A good rule of thumb is a flat headphone should be straight flat to 500hz, and then have a gradual 10db diagonal slant down to 20khz. A little bit of a treble spike around 10khz can be nice to make the headphone sound more balanced and bring out micro-detail plus act as a sharpening filter to the attack and treble elements of instruments, but too much of it is a bad thing.
Higher impedance just means it requires more voltage swing to obtain the same volume, and sometimes higher impedance headphones have large impedance spikes that can be 1-200 more impedance higher centered around 100hz than their average impedance, which if not properly powered, can lead said headphones to sound thin.