First of all, I would like to stress how amazing flat stomachs can be. You can have everything going for you: pretty face, nice ass, and nice boobs, but if you don't have a flat stomach it could potentially ruin all of that. The opposite is also true. You may have small boobs and a small ass, but just having the flat stomach wins you so many points.
I also personally think a flat stomach is a good indication of physical upkeep, fitness, and health. This brings me to my other point:
I know this is a really controversial topic on this site, but I don't want thin people to be left out of any beauty categories. You may or may not be struggling with that, but I find women on the slender end of the spectrum to be very attractive. I don't mean to attack anyone or say people are overweight at all. It is just a personal preference--I, more than most guys, and more than is normal, find thinner women to be very attractive. However, it is also a turn-off if what little weight they do have its very untoned and mushy. If you're going to be thin, it may as well be soft and toned.
But back to my original point, even a thicker girl can still be super-attractive simply by having a flat stomach.
Besides the face, I would rate the most important features of a woman as:
6. All the rest.
What do you think? Is a flat stomach too hard to accomplish for most people? I seem to seem to see very few of these glorious beings.