When it comes to making cherry crisp, the best cherries to use are sweet cherries. Specifically, you can use either Bing cherries or Rainier cherries, which are the most common varieties of sweet cherries. These types of cherries have a firm texture and a sweet, juicy flavor that works well in baked goods like crisps. It's worth noting that sour cherries, which are also known as tart cherries, are often used in cherry pies and other baked goods. However, sour cherries are not typically used in cherry crisp recipes. Their tartness can be overpowering and may not pair well with the other ingredients in the crisp. Montmorency is the most common variety of sour cherries you'll find. If you're using these tart cherries, you'll want to increase the amount of sweetener in this recipe.