Basically every app that is related to a proprietary service. Amazon, Battle.net and Steam authenticators, banking apps, Spotify, etc.

There already was a post like this this year but now my answer is "a standardized push notification system (most likely federated) that's actually possible to be implemented in a user friendly way". Google doesn't want to encrypt theirs afaik and apparently some people are concerned about the traditional "every app is responsible for its own notifications" approach consuming much more battery, even though I didn't notice it myself (I guess it's possible if you have 50+ apps installed but it's not something that should be a thing in the first place).

Android Auto

The default Samsung messages app. It allows custom backgrounds for each text conversation. All apps I find only allow custom colors, no custom wallpaper. Eben Google messenger had this feature... Then they took it out and replaced it with pre selected 'color themes'

I'm actually pretty happy to be using mostly FOSS apps. The exception are banking or services apps, which I'd never expect to be available as open source.

That one DAW for electronic music... The logo had a hexagon or something.. Caustic maybe?

Termius

Not just Android, I want a cross-platform ssh client that shares keys. Termius is probably overkill for that, but I haven't found anything else that works on Linux and Android. The real issue that made me stop paying for it is that for rpm based Linux I have to use the snap version and snap is buggy as heck with multitasking.

All these mouse cursor touchpad for big phones-apps. They seem pretty easy to do and are quite handy.

Strava, komoot and a calorie tracker that's actually decent

Picsart. I'd like something that can do a bit of photo editing, adjust brightness/contrast/curves, work with layers, and conveniently slap together collages, but that doesn't interrupt me in between every other operation with an ad or a request to sign up for a subscription to the app.

Maybe Niagara Launcher, though I'm quite happy to pay the dev a bit of money (not required for most stuff actually, I only login on my phone)

Until recently I'd have said Symfonium for music playback from Jellyfin, but the Finamp beta gave me an OSS alternative.

Ideally banking apps, booking.com and TripAdvisor all had FOSS alternatives, but that's not realistic.