Who said “ flat freq response “ is the best?

You need to check out the Fletcher Munson curve. Our brains perceive frequencies at different levels, depending on the volume of the sound. Most people do not prefer a flat response. I set a room with equal volume in all frequencies about 30 years ago. It sounded very "flat," no pun intended. 

Of course without a flat response any attempt at neutrality has gone out of the window. So it makes sense that all loudspeakers/ headphones should aim to have a flat response as possible as long it's not at the expense of dynamics.

Some like me, felt that too many British loudspeakers in the past with a reasonably flat response tended to sound too polite. For me that's too great a price to pay. You may feel different.

On the other hand too great a deviation from flat gives the speaker an obvious character on everything you play and you will hear this constantly.

It doesn't have to be ruler flat, the main thing is to avoid obvious spikes. Especially if you are mixing/ broadcasting/ or in any other pro use. Pro's do tend to stick with what they know.

For the rest of us some form of tone controls / equalizer is always desirable to compensate for different rooms or recordings ect.

As usual with loudspeakers, nothing is close to perfect. The best you can do, as ever in audio, is to pick your own set of compromises.

Flat freq response for what and for whom? An anechoic chamber? A high end audio show room? Tom, Dick or Harry’s room. Every room is different. And for whose ears? give me a break!

A flat frequency response means all frequencies are reproduced at the same amplitude. Even if that was somehow possible to achieve in a listening room, that would remotely resemble what live music sounds like. I think a better use of this measurement in as far as comparing speakers is concerned, is how wide of the frequency spectrum the speaker drivers can cover and not if there are ripples within that response.

There are some other important technical parameters to consider. Dynamic range, sensitivity, response to square wave, distortion. Frequency response is probably fourth or fifth down the list.