With our current technology it's impossible to make a flat frequency response speaker. It's possible for amps, mics, DACs, but not for speakers, unless you talk about million dollar facilities.
So you need to pick from different non-flat speakers, which means listening test if you really care. You can't figure out which one you'll like just from looking at the frequency response graph.
Flatness is a curve, not a number. If you try to distill it into a number (by integrating the error for example), you can have two speakers with the same "flatness number" which cost about the same, yet sound very different.
This is not theoretical, I've experienced it - two professional speakers (or monitors as they are called) from different manufacturers, which cost the same and the frequency plots look the same to a human eye if you put them side to side, but sound very different.
Which is why it's preferable to listen them if you really care. But there are "safe" recommendations for every price range.
So you need to pick from different non-flat speakers, which means listening test if you really care. You can't figure out which one you'll like just from looking at the frequency response graph.