521 - well, that is quite the question. I shall answer thusly -
The first and most important part of any pie, is the crust. It's all got to do with the balanced interaction of gluten and fat making the perfect not to soggy not to hard crust. The underlying difficulty in the perfect technique comes during the first stage of cutting the butter into the flour, but to digress you need to know what your doing with the ingredients if you want to make it perfect. Lots if cooking books get this all wrong all the time.
Secondly it's all about the filling, to which the world is your oyster. Anything pie. Once you have the perfect crust method down, this is the easy bit.
Consider pie, a gift from the beyond. They taste so good because there is something for everyone.
Except apple pies. They are evil and not good. Eat cherry pie instead.