The main diet or staple diet for cockatoos is the food that is needed for them to live a healthy life. All other foods are just extras. It is divided into two caterogries; dry foods and fresh foods. This page is about dry foods, you can find the page about fresh foods here. One in not complete without the other, you cannot exclusively feed dry or fresh foods to your cockatoo.
Seeds or pellets
As a main dish you can feed your cockatoo seeds or pellets. A mix is not recommended as your cockatoo will pick and choose.
Pellets as food for cockatoos
Pellets are uniform pellets made of ground seeds, nuts, protein and with added vitamins and minerals. They are made specifically for feeding to parrots or cockatoos. Your cockatoo cannot choose which part of the food to eat, because all of the pellets are uniform. Therefore he has to eat all ingredients of the food in the determined percentages instead of selecting only the tasty parts, thus obtaining a balanced meal. The only problem is that if the manufacturer does not select his ingredients well, your bird has no option to choose something else to eat. Also it can be argued that eating the same food every day can become boring for your bird, even though you feed him fresh foods as well.
Seed mix as food for cockatoos
Instead of pellets you can feed your cockatoo a mix of seeds. Many manufacturers have mixes of seeds especially made for cockatoos that can be bought in pet shops. Ideally the mix of seeds is determined scientifically to provide your cockatoo with all his nutritional needs, besides fresh foods like fruits. However, you can also find many seed mixes that are poorly designed and contain mostly cheap seeds for the manufacturer instead of nutritional seeds. Do not buy seed mixes with a lot of fatty seeds (like peanuts, sunflower seeds and nuts) or seed mixes with a lot of inedible seeds (uncooked corn and uncooked grains are inedible to cockatoos).
If you give you cockatoo too much of this seed mix, it does not matter which seeds are in there or how diverse the mix is. He will exclusively eat the seeds he likes and leaves the rest. This makes his diet very limited and can result in nutritional deficits.
This main food, pellets or a mix of seeds, is the most important food of your cockatoo but cannot be fed exclusively. It always needs to be accompanied by fresh foods like fruits, vegetables and others foods. Here you can read all about that.