Who said “ flat freq response “ is the best?

To this point, no one has mentioned phase.

Minimum phase error trumps small deviations in frequency response in contribution to a realistic and non fatiguing presentation six ways to Sunday.

It is a great failing of the audio press that phase has been largely ignored for the half century since Richard Heyser's seminal work.

IMO, phase has been ignored because so few people have heard a properly phased system. Designers who don't understand the importance choose to ignore it and the media play along. Few systems are phase correct to begin with and those that are rarely setup correctly. Very few people who buy HiFi ever hear unamplified music in a good hall and thus have no reference to an acoustic space.

The Absolute Sound recently reviewed a time-aligned 2-way loudspeaker with 1st order crossovers. Unless the drivers are perfect, it is simply not possible to achieve minimal phase error with that design.

The AES has an anthology of Heyser's work on Time Delay Spectrometry.
It's long @ 279 pages: http://www.aes.org/technical/documents/openaccess/AES_TimeDelaySpectrometry.pdf 

Stereophile opined "Essential reading for the informed audiophile: the AES anthology of the late Richard Heyser's writings" on https://www.stereophile.com/content/2011-richard-c-heyser-memorial-lecture-where-did-negative-freque...