For snacking, all apples are good, depending on your preference, of course. But for baking, you have fewer choices.
For baking, you need firm, crisp, high-starch apples that won't get mushy when you put them in the oven. Texture is key. Soft apples will not do well. You should also cut the apples into different sizes for variety of texture. It is also a good idea to mix sweet and tart apples or use sweet-tart ones to create a more complex flavor. Keep in mind also that sometimes an apple that is good for baking will not taste good raw. Cooking as well as the other ingredients you use change the flavor and texture.
The tart Granny Smith is a favorite baking apple. Its texture holds up well to heat, it is both sweet and tart, and it is easy to find. While some like the crisp skin for baking, Cooks Illustrated suggests peeling the apple before baking to prevent mushiness. Other good baking apples include the tart Jonathan and McIntosh as well as the sweeter Gala and Golden Delicious.
If you are making a pie, you have a few more choices. Try Braeburn, Empire, Fuji or Ida Red. If you can find them, try Orin apples for baking or pies. They are very sweet with a complex flavor suitable for cooking -- and they won't get mushy. Other apples have their fans -- and opponents -- for baking. The ones mentioned here are widely recommended. Remember that freshness and firmness are crucial, no matter the variety.
One pound of apples yields 2 to 2 1/2 cups of sliced or chopped apples. If a recipe calls for 2 large apples, you can use 3 medium or 4 small.