Use Granny Smith apples, as most other varieties are likely to produce a weaker version of this soup. You need acid and flavor, and Granny Smith delivers both. Try to find organic apples, then leave the skins intact as most of the flavor and nutrients are found in or just under the skins of the fruit. Use your imagination when it comes to garnishes: Consider chopped, toasted walnuts, grated sharp cheddar cheese, or a dollop of sour cream or yogurt.