Read Era is technically free, but I paid for premium years ago and have never regretted it. I can open any kind of uncorrupted book file, from the Amazon reader format to PDF to epub, and everything else I've ever come across. It has a great search function, and the ability to file a book into a custom 'Collection'. You can edit the details of a book, like adding Author or pusblisher info, add your own personal notes to a page or highlighted quote, see an aggregate of all your highlights in a particular file, and adjust the font, background color, and contrast to your hearts content.

I make my whole family use it now, cause I love it so much and Premium works on Family share.

Monument Valley. Got the first one for free during a promotion but loved it enough to pay for the sequel and extra levels.

Cryptomator is a fantastic way to securely upload your stuff to cloud storage providers like Google Drive, OneDrive, etc. In my case, I use it to have an encrypted blob of my stuff with me on a drive when I'm out and about.

They also give you the ability to purchase a license independent of Google Play if you didn't want Google to get a cut.

I use Paprika 3 extensively.

I find recipes online, download them to the app stripped of all the online recipe bloat. It sorts all the information automatically, including notes and nutritional info. I can check off ingredients and highlight directions, edit tags, compile menus, add my own notes and write my own recipes, it automatically provides a grocery checklist, has a serving calculator to adjust amounts for whole recipes, built in timers, and that's just the basics off the top of my head.

It's free up to a certain amount of storage but I think all the features are available.

Threema!

Balatro

I love Simple Audiobook Player+. The UI is super minimal (and really maxes out the whole OLED black thing if you choose it) without compromising on features that are kind of essential for audiobooks (e.g. delayed pause/sleep timers, speed settings, volume boosting, an EQ). My favorite thing is the "undo seek" button. I'm an oaf who is constantly inputting accidental touches. When I was using Audible, I'd have to manually find where I was after accidentally hitting the next chapter button or moving the dot on the progress bar. SABP lets me just undo that shit.

It hasn't been updated in a while, but it doesn't need updating when it does its job so well. There are no ads, no marketing notifications, just books. It's like a program from coreutils in app form. It might be a bit ugly or outdated looking, but I'm about that.

Torque and a $5 BT car computer dongle. It tells you everything about your car. You can see what warning lights mean and clear the codes.

Tasks.org is a wonderful open-source todo/task app, that has a low-cost monthly subscription to use it's syncing ability. It's worth it to support FOSS wherever we can.

Tasker. Basically an interface for writing scripts for your phone. Even if you don't have a use case in the beginning you'll start finding things to do with it.

Stardew Valley

Sleep as Android

It's just a really great alarm clock app, but with tons of other sleep tracking functionality. I've always had trouble sleeping through my alarms, but I never do with this.

Slice and Dice is a very entertaining one time buy game. No bullshit in game purchases, no ads, I think developed by a singular guy.

Symfonium is an awesome music player that's a one-time $5 purchase.

Great question, btw.

Others have recommended other file explorers, but I use FX and rather like it.

MiXplorer: Tabbed file explorer with many features. You can get it for free from their website, but it's available paid on Google Play.

Symfonium: Music player compatible with many backends, such as local storage, WebDAV, Subsonic (which includes Ampache, Navidrome)

aCalendar+: Calendar app with many widgets. Best part is the persistent notification, which shows what's happening today, and will happen tomorrow.

Cryptomator: Cross-platform file encryption program, also open source.