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!
Dice by PCalc - For when you need to roll some dice
Blackbox - It got some hype when it released but if anyone still hasn’t tried it, it is highly recommended. You will need to learn everything your iPhone can do and think outside the box to solve these puzzles.