Bird feed differs per species. Different birds require different foods so by supplying a variety of foods, you will attract many more species. Household scraps are enjoyed by many garden birds while fruit is popular with thrushes and blackbirds.
If buying bird-specific food, find a combination of peanuts, seeds and live food to attract the most birds. Good ready made mixes will contain sunflower seeds, broken peanuts, flaked maize and smaller seeds such as millets.