Granny Smith is my favorite type of apple to use for apple crisp. These apples keep their shape well even after cooking. Their flavor is sweet/tart, which is a nice balance to the sweet topping. However, you can use any type of sweet/tart crisp apple you like. Northern Spy, Braeburn, or Jonagold are all good options. If you use an apple that’s on the sweeter side, add 1 teaspoon fresh lemon juice.