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!
If you’ve set up a Plex library, PlexAmp can also play FLAC files but it can also stream them so they don’t actually have to be downloaded to the phone. It does have that option, though, if you want to play them offline.