These are the apps that I use the most frequently on either my iPhone, iPad, or PC:
Smart Record: I use this to record ideas and make field recordings.  I just found it has a transcription feature as well, which I’ve never used.

Pro Metronome: This is a free metronome and I’ve used it for years.

Tuner Lite: another free app that I use a whole lot for both practice and teaching.

ForScore: This is an invaluable app for musicians, and when used with the Apple Pencil it is amazing and multi-purposed for anyone using PDFs.

iRealBook Pro: I use this on gigs and for practice, and has saved my butt on numerous occasions on jam sessions when I don’t know the changes for a tune.