I always recommend using an apple you enjoy eating, and for me that's Gala or Golden Delicious (also my apples of choice for apple turnovers recipe and apple butter). Keep in mind that firmer apples are best for baking, so I don't recommend McIntosh or anything similar in your apple crumb muffins. You could use Granny Smith apples, but I prefer to save them for apple pie since they're so tart.