I have been wondering for a while, after asking different people and without any of them providing what I would call an \"at least a bit concrete answer\":
Quest
I find that in most instances, AppDelegate provides a good place to place some base functionality (such as a a background image you want applied in every controller), but that you'll want to have additional controllers and model code elsewhere. A navController or rootController would often be placed as a property on your AppDelegate.
So, I would say that is somewhere between "neither" and "controller", but leaning more towards "neither". Definitely not "model"!