Recently it's been Obtanium, basically it allows you to add apps which are hosted on GitHub/other app sources which don't have auto updating, and have them automatically check and offer to update!
It's been really useful for Lemmy clients (liftoff/Jerboa)/any android apps from:
How is the battery drain for you? I'm under the impression that it destroys mine, even though I set it to only run when charging, so I often end up killing it.
I was looking for a file manager because I was using Google Files or whatever is called and it is way harder than it should be to access all the folders in your Downloads folder... Simple File Manager seems to work quite well for this.
Also the music app since it seems that Android has not a stock music app, or that is what it seems.
I unfortunately had to use an iPhone as a temp phone, and it was not a fun experience trying out other manga/comic readers and none ever came close to Tachiyomi.
This app alone is one of the huge reasons why I prefer Android. I love that I can natively run a bunch of Linux utilities directly on my phone and use it like I would a regular computer. Everything from Vim, Python, and various networking utilities, I've played around with so many different tools which has helped expand my skillset over the years.
I also use the Termux-X11 addon to have a full xfce desktop environment running on an external monitor with HDMI out over USB-C.
Phones are quite capable devices and using tools like these really unlocks their full potential.
Firefox by a huge margin, because it's integrated into how I use the internet so well now I don't even notice it's a FOSS app.
As far as more niche apps goes, Joplin. Since they added WYSIWYG editing it's an easy Evernote replacement that just does what I want in a note taker/web clipper app.
I use Fastmail for my email and calendar.
Their server also supports CalDAV task lists, but unfortunately they don't have any web UI for tasks. In the past, I have self-hosted Nextcloud for tasks, but ultimately dropped it as I had some issues with my server at the time, so it wasn't reliable, and I found Nextcloud to be too heavy and slow for my needs. I'm now considering self-hosting Vikunja.
Sorry for my boring answer. Never really thought about it until now. It'd probably be oandbackup.
It allows me to make my phone fully operational, customized to my liking. Including replacing the stock ROM with a custom one, rooting, and installing my favorite apps *with their settings*, in less then 15 minutes.
More recently Habitica and Goodtime have been instrumental in the way that I approach productivity
Habitica is a habit tracker/to-do list that lets you treat your life as an RPG, and provides me the extrinsic motivation of "I want to level up" that pushes me to accomplish tasks that I otherwise would have been lax on
Goodtime is simply the best pomodoro app that I've found that has a minimalist interface, configurable timers (on the fly), and robust statistics monitoring (with the option to export to CSV)
In combination they've helped me move my daily focus into more deep and focused work, rather than trudging around and getting distracted by whatever catches my eye
Can you talk a little bit about how you use Habitica? I've tried it and can't seem to make my brain care even though I fall into games easily and even though I know it would be good for me.
I would've said Infinity for... something, but we know how that turned out. So, I'd say it ir most likely Fennec F-Droid, it is the first app I install in every device that isn't a tablet