The best type of cherries to use for baking pies are sour cherries, such as Morello and Montmorency cherries. Those that have red, unblemished outsides and the stem still attached are best.
Most of the store-bought cherry pies and cherries that come in cans are the sour variety. That is because the tartness of the cherry lends itself to baking better than the sweeter cherries, which are better eaten by hand as is. Most stores do not carry fresh sour cherries, so individuals looking for fresh sour cherries should check the local farmer's market. Sour cherries tend to be very juicy, so adding starch to the recipe to thicken the cherry juice is often necessary.