Whatever you decide check the label and make sure the list of ingredients adds up to 100% (same with the list of analytical constituents). If it doesn’t (I’ve just checked one of the better quality foods, mentioned by a number of posters on here, and it the ingredients only add up to 68%) then you can only speculate what the missing ingredients are. These missing ingredients are ones that can be changed without you knowing, so you can find something your dog has been happy and healthy on for months can suddenly upset your dog and it no longer wants to eat it or worse the new unlisted ingredients cause health problems for your dog, usually itchy skin or a rash which can involve numerous expensive trips to the vets to try and determine the source of the problem which you can never pin down to the food as the ingredient which is the culprit is not listed.
Also try to keep away from foods that have a lot of E numbers in their additives list.
If you’re having trouble sleeping and want something to read you could check out the pet food legislation http://www.pfma.org.uk/uk-pet-food-legislation
Beware forums (everyone believes they are feeding their dog the best food) and pet food comparison sites (which often have hidden backing from manufacturers so are not totally unbiased).
Best advice I can offer is check the labels first and once you’ve found a food where all ingredients are listed, that your dog enjoys and is healthy on, and that you find affordable, stick with it!