KDE Connect is an app to connect your phone and computer. It works on most OS (windows, Linux, etc.), and it works extremely well (better than the solution developed by Microsoft).

FairEmail: I hated reading email on my phone, till I found it. I was so happy that I already paid for it 3 times (optional!) just to give something back to the dev.

Waze: it belongs to Google now, but I like it much better because it's like a social network for navigation and maps. You can also edit the maps by yourself. Another advantage is that it shows speed limits and warns you about police while driving.

A few of mine :

• AndOTP : A good app for OTP that actually let's you backup your keys and move them easily and securely to a new phone
• Blokada : A system wide adblocker for Android. 4000000 ads blocked so far on my phone
• YouTube Vanced: Not really a "hidden" gem but a cool app nonetheless. I wish people knew you could get an ad free YouTube experience on mobile. It also includes sponsor block so you really get straight to content. Also let's you get a YT popup player and shutdown the screen with the video still playing.
• F-Droid an Android open source app market. Often includes apps that are not allowed on Google market.

• NewPipe has made my YouTube watching experience way better; it's a standalone app with features like ad blocking, subscriptions, and downloading.
• F-Droid (particularly the Droid-ify client) is an open-source app store that generally provides quality apps.
• Obtanium has helped me stay up to date with some of my more obscure apps, which are released only on Github. Obtanium can auto-update apps from a GitHub link or similar, which makes the process way easier.
• Auxio is a pretty good music player, it's quite lightweight but it also looks good.

All of these are available on F-Droid, I find that many F-Droid apps provide a better or cleaner experience than Google Play apps.

Mixplorer - aka MiX, the only file explorer I've ever needed. It's intuitive enough for basic file browsing, but it can do heavy lifting tasks. I exclusively move files on and off my phone by starting an FTP server.

Pulsar - One of the only apps I've ever paid for, it's a music player that does everything I want and nothing I don't. I'm one of those nuts who still maintains a personal library of thousands of songs rather than using spotify or whatever.

MS Swiftkey Keyboard - I went looking for this when Swype was finally discontinued. Gesture typing wasn't common yet and I still find google's stock keyboard absolutely insufferable. Swiftkey lets me adjust the size and layout of the keyboard with far less restriction.

For me

1. MiXplorer (Mix) : Clean and powerful file manager at the same time. It's easy to use for normal user and very powerful for power users.
2. Lookup Pro: Floating dictionary that can blend in to native Android context menu when highlighting words. It's similar to iOS lookup or Kindle lookup feature.
3. Maosure: Google virtual measure app that make use of ARCore library.
4. Moon+ Reader Pro: favorite ebook reader
5. Seal: Favorite youtube downloader. Unlike other YT downloader that is riddled with Ads, it FOSS app.
6. Toolwiz Photos: Best all in one photo editor for me. It's very underrated. The variety of feature it pack is u match by any photo editing apps for me. From color correction to healing and cloning tools, the fun feature that gets old very fast like face rater, face swap, pip, etc. I just noticed that it has AI painting feature too.
7. TTS Server : Azure/Ms Edge TTS ported to Android to use it as system wide TTS. It's the most natural free TTS I've ever used. It's originally Chinese only app, but I forgot where I got English version from.

What phone do you use? I recall my call recording app stopped working a couple years ago due to some Google permission thing.

Shameless plug but I am promoting my app here on lemmy. Check out my profile if y'all want 🥲

Bitwarden - Allows me to have unique, strong passwords for each of my profiles without worrying about forgetting it.

Aegis - Great open source 2FA app.

Standard Notes - Great note-taking app. I have it installed in all my devices which allows me to seamlessly switch between devices.

TrackerControl - Block all the trackers in the various apps installed in my phone.

Plees Tracker - As someone who has been trying to correct their sleep cycle, this app has been invaluable for me. It is an open source app that allows you track the duration of your sleep daily and then makes charts from it.