You want to use an apple that is firm for apple cake so that it doesn’t disintegrate into the cake. You want to be able to see some apple chunks after baking. If you like tart apples, go with granny smith. If you like sweet apples, go with gala. If you like something in between, go with honeycrisp or pink lady. You can also use a combination of apples for a mixture of sweet and tart.