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!
VoiceDream and SpeechCentral are amazing text to speech apps that turn your documents (PDF, txt, ePub, Mobi, AZW3 into audiobooks that let you read by highlighting and adjusting the speed etc.
I don't know if VoiceDream can still be bought on iOS but I know SpeechCentral is almost as good and it can be bought one-time.