Preferably lesser known but game-changing apps that are able to be bought one-time and put all others to shame.
To help clarify your thinking, which apps have produced such an outrageous level of value (regardless of one-time cost) to the extent you believe it should be #1 in its category, not necessarily #1 app ever.
We'll do a seperate thread for Mac, Windows, Linux, Android, etc but let's stick to iOS for this one. Thanks Lemmings!
RecipeBox is another good one that has a recipe search function and a grocery list function that connects to your recipes and populates what you need directly from the recipe itself.