Who said “ flat freq response “ is the best?

There are no good reasons for an electronic component to not have flat frequency response.  Loudspeakers have a different set of issues revolving around expected room interactions.  Flat frequency response at the listener position is not desirable.  It's sounds tipped up in the treble and unnatural.  Most listeners would prefer a gently downward sloping frequency response at the listening position.

"Flat" means a component or speaker is not likely to color the sound on its own. The problem is that there are many different components in the system...and then there’s the room itself...and your own hearing, your mood, etc.

The problem is not so much the individual component or speaker. The problem is in trying to get all the different freq. curves (each with their own slight variations) to work together and add up to one final in-room response that you can live with...flat or otherwise.

The big lie is to think that because you’ve bought a component or a speaker with a "flat" response (or in fact any stated response) that therefore you will be assured that it will remain so in your system and in your room. It isn’t so. The room may dominate, other components may dominate. Bad interaction with certain product combinations may dominate. Even things like a lack of power-conditioning may dominate. All you have bought is a component that will ’allow’ for a flat response under certain conditions (sometimes those certain conditions are specified and sometimes they are not), but the flatness is not guaranteed in your setup, only on the test equipment used to test that piece of gear. But, "most" gear is designed to be used successfully in "most" rooms and with "most" other gear.

A truly flat response may actually be audibly (subjectively) desirable, but almost always only with an Extremely well-executed system of rather expensive, high quality and full-range gear. In fact, it might surprise a lot of people just how much attention to detail and how much money must be spent to subjectively feel safe from any real concern with frequency response issues altogether. For the rest of us, we will need to consider it - whether a little or a lot.

I think about stuff too much too.

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!