Per other commenters, Granny Smith gets downranked for being a mediocre apple for eating plain in-hand, but it's great for using in salads. We have a Granny Smith apple tree and we dry all the apples in a dehydrator and it makes the best apple chips.
Granny Smiths are also good for baking in general. They have a twang to them that offsets some of the sweet, as well as being a crunchy apple that doesn't turn to mush during the cook.
