Which spirit would be the best substitute for calvados

I haven't made this recipe, but I'd suggest sherry - I've braised pork chops with apples and sherry (onions in there too) and they work very well together.

I see one of the commenters on the recipe tried bourbon, and gave it raves. You would seem to have plenty of apple in the recipe anyway; I wouldn't worry about it much- brandy, dark rum, bourbon- whatever you have would probably work fine.

I would also use cognac or armagnac.

Hard Cider made from apples.