What’s that one app on your phone that is utterly invaluable to you specifically. Maybe it’s got a super small use-case, but for you, and you alone it’s absolutely invaluable.
I’ll get us started…
It’s a small all that allows me to stitch photos together. I usually use it to stitch screenshots together. It’s got a nice interface and is absolutely fantastic at the one thing I need it to do.
What about you?
Findroid - For watching and syncing my Jellyfin library to my phone to watch films and TV on my commute.
Newpipe - I use to sync YouTube videos offline and background play etc.
Spectdroid - I’m a sound designer and having quick access to spectral info is great.
Mini Piano Pro - Always using this to check what key a track is or what a note is.
Fitbit - I use this to track my macros for my weight training.
Kiwi Browser - Browser with Adblock and chrome plugins. I guess I use this a lot to access the front end management apps on my home server.
Genius Scan - Really good pdf scanner.
Tuner (Boss) - Guitar tuner.
Total Commander - Good file manager for my network.
Termux - Terminal emulator.
I think those cover the jist of the question. I like apps like Ampere but don’t think they quite fall into the category you’re enquiring about. I imagine the niche side of your request would put Spectdroid as number one, a bunch of the others are pretty obvious after rereading your question.