>>510285606
Apple variety is probably the most important. It needs to be firm and juicy and not mushy (to hold form after cooking), and taste should be sweet & sour. Just sweet would result in a bit bland taste.
It can be improved with lemon juice, but it's still better to choose the right kind.

That filling can also be used for good pancakes, or rather 'crepes' as they're called in the west.