Who said “ flat freq response “ is the best?

Speakers need to be "voiced," i.e. you have to listen to them. I talked to a speaker designer years ago who said that if you had the ability to adjust the various sonic parameters of a speaker from your listening spot (including cabinet materials, crossover settings, speaker materials), the results you would wind up with from simply listening to music would be far from flat. 

I think most music lovers and audiophiles can agree that there is no such thing as “flat” anything - especially speakers. Although, it is probably a good thing not to have prominant peaks or dips in a speakers’ response. That said, I believe that the key ingrediant to great sound is the room itself. Assuming your equipment is well matched, and you are at a certain price point, you can get more bang for buck in room tweaking than in component changes.
I have 2 systems that I listen to frequently.  The one with superior room acoustics sounds better, no matter which equipment I use in it.

Come on people. Speakers’ frequency response is almost always specified along the lines of 40 Hz to 16 kHz on axis + - 2dB. Their sensitivity is almost always specified along lines of 92 dB for 1 Watt at 1 meter. Just like stereo cartridges are usually specified along the lines of 15 Hz to 20 kHz +- 1dB.

@geoffkait 

For everyone,  geoff is accurate,  speakers publish their spec's with a frequency response and a variance within that response. 
Exceptionally flat speakers are normally rated Plus or Minus 1 db.  Anything that is typically more that plus or minus 3db is normally not considered audiophile,  although that certainly is not cast in stone. 
Today,  Many speakers are rated at a flat 2.83v input rather than 1 watt 1 meter.  For accuracy,  you need an accurate impedance, an accurate response curve and an accurate sensitivity rating.