OK guys this is a VERY contentious issue and it will require significant research and self education on your part. If you love your pet, it is worthwhile. Firstly please do yourself a favour and read the book "Food Pets Die For" by Anne Martin. It will do more to educate you a two hours of reading than you could get from online all day. Her sources are vetted and I have personally confirmed with veterinarians that what she says is correct. A couple of the high points that you can research.
1) Low end pet foods contain rendered proteins from rendering plants. Those rendering plants are allowed by law to include decomposed pets from vets and kill shelters as well as diseased farm animals and road kill. The rendered meat is then boiled down and ground and then dried and pelletized into pet food. It should be noted that the drug used to euthanize pets that are going into this rendering process survives the heat of the rendering and can be found and has been found during tests. This has caused Vets to have to up the dosage of these drugs because pets have been growing immunities to the drug.
2) Cancer causing agents have been found as well as heavy metals.
3) Various pets foods like Eukenuba, Iams, and Purina have all been recalled for Corn meal mold that killed thousands of dogs a few years ago. The pet industry did not recall these products until there were already hundreds of deaths even though they knew it was going on.
Watch out for things like "meat meal" and "meat by product" these are the two biggest culprits. The better foods do not use this. There is no easy fix and it's not cheap to feed your dog healthy dog food from the store. We feed our Lab's a combination of a high end food with a homemade diet that we make in our crock pot. vegetables, Chicken, Vitamins, and Eggs. The combination of a high end food with supplementing makes the per bowl cost about the same as a medium cost dog food. One fact that many people don't realize is that before we started feeding dogs pelletized dog food in the 50's there were dogs that lived 17 to 20 years. Just think about that almost twice the length of life.
My best advice is find yourself a good holistic vet someone who is not in the pocket of the huge pet food companies and work with them to design a good diet for your companion. This is such a sensitive subject and one that most people don't want to know about. Ignorance is bliss for everyone except our pets.