This is one those things that is very individual and trial/error things. Among us you will find parents who have had success with girlfriend or girlfriend/CF diets, dye-free diets, you name it. In my case, my difficult child seems to react to something(s) in processed meats and does better on whole grains (including wheat), organic milk, and added Omega-3 supplements.
Keep a diary, let that diary include moods, foods, medications, moon phases, known allergens (if your difficult child has ragweed allergies and it's that season you expect issues). You never know what connections you might make, because allergy testing doesn't catch everything. Some kids have reactions to artificial sweeteners or excess sugars, and some don't.
There's a lot of research about GMO based foods (you might be surprised how many foods we think of "healthy" fall into this category) that's rather scary. I think organic is healthier in general but obviously it's simply not possible for even most of our food to be organic, and Kiddo's biggest protein source is milk so that's where I concentrate our budget for organics. Since she can't have processed meat I've found things she considers to be viable substitutes (also expensive but worth it), such as vegetarian versions of bacon and corndogs, and preservative-free lunch meat (Hormel makes this, their Naturals line). Living in a small town we don't have access to a place like Whole Foods, I wish we did, but I do what I can within our means. We use flaxseed bread to get more fiber and omega-3 in her, and Jif's Omega-3 peanut butter, too (JIF is the only brand she'll eat). We use whole grain wheat pasta, Kiddo was raised on it and thinks the typical "white" pasta tastes funny
You'll have to figure out what works (and doesn't work) for your difficult child and your family. Expect to stumble and make some mistakes along the way, it's natural.