I like to use a crisp and tart apple, such as the Granny Smith. They are a more tart apple, but because the topping is so sweet, it balances nicely. They also hold up very well in the oven while baking. However, if you are worried about them being too tart, mix in another variety that works well in baking, like Honeycrisp. I wouldn't suggest an apple that is on the sweeter side, as it will make the whole dessert quite sweet. Other great options include Braeburn and Golden Delicious. I suggest avoiding Red Delicious or Gala.