I personally prefer Bosc pears (sometimes called Kaiser pears) for baking and poaching because their firm, sweet flesh retains its shape under exposure to heat. Anjou or Concorde pears are also fine options. Be sure to choose smooth, unblemished fruits that are mildly fragrant. Ripe baking pears should yield to gentle pressure from your thumb, but should not feel soft. If your pears are underripe, place them in a plastic or paper bag with a banana for a day or two - the ethylene gas from the banana will speed up the ripening process. If your pears are overripe, they might not be great for this pear tart, but they would be perfect for turning into a sauce or syrup.