OnSong: I use this to organize all my music PDFs for Masses, worship nights, etc. This is definitely the easiest way to use iPads for music, since it connects with Dropbox, SongSelect, and Planning Center. If I download a SongSelect chord chart, I can transpose the key easily without having to download a new chart. Even though you have to pay extra for the add on, it's worth it to be able to annotate and draw on the charts.
iMissal: It's nice to have an app that can pull up the readings for the day without needing to wait for it to load or wifi. It's come in handy during Mass when I need the Gospel Acclamation verse.
Ordo: This is the app version of the book made by Paulist Press. It has critical information like should you sing the Gloria, is there a Creed, what other reading options do you have, etc. It's a terrible interface unfortunately, and you have to download a new app every year, and it's very expensive for an app, but I justify buying it since it's equivalent to a book.
Catholic Bible: My favorite Bible app is made by Just1Word, it's good to be able to copy and paste scripture readings as needed.
Planning Center Services: This is an online service for scheduling musicians and distributing rehearsal sheet music and audio files to practice along with. The phone app interface for listening and practicing with music is exceptional. It's a monthly fee for the entire service, so I'd take a look if it's something you can budget. It's saved me about a days work per week. In the future, if we switched to all iPads instead of binders, I would look at their Music Stand addition since it can sync all the music between the iPads. It also has a built in alternative to ProPresenter.
Google Sheets: This is how I plan music for Masses. I create a sheet for every year so I can go back and look at music from years ago. I enjoy being able to share the planning spreadsheet, make comments, and access it from the app.
GuitarToolkit: This app is amazing, since it has a tuner, scales, metronome, and chords. I always use it when I run into a really weird chord, since it gives you many different positions for playing the chord. Also, you can input notes on the fretboard and it'll tell you what chord you're playing.
Tempo: Great free metronome app. It has a tap tempo (gives what tempo you're tapping) and allows you to store tempos.
Subdivide: This is only if you're looking for a complex metronome. It's the app version of a Boss "Dr. Beat". I use it a lot since you can blend the different types of subdivisions, as well as a tap tempo.
Garageband: This is great if you want to create rehearsal recordings for your groups on the fly. Typically, if I'm not using my full Pro Tools setup, I'll just use the Voice Memo or Music Memos app on my phone though.