UI = User Interface UX = User eXperience
Most open source apps offer out-dated or classic material design UI. If the apps works great and the UX is not terrible you might find it okay. But intuitive UI are better, and modern looking ones are pleasant to use, sadly they are not the majority. Here is my list of Beautiful Open Source Apps for your android device !
Also look at : The ultimate F-Droid setup for more open source android apps. and Beautiful Open Source Apps for iOS for your iOS device
Nice UI comes to personnal preference. So this list is only based on my taste of what is a good modern looking app.
While open source is great for privacy and security it is not perfect. So keep that in mind before downloading any apps. I again encourage you to do your own research about them and read their Privacy Policies.
I am not a privacy and security expert. I'm also don't consider myself as a developper so I do not have any technical knowledge and so I didn't look into any source code of any apps in this list !
Aegis is my go to for 2FA based on TOTP. The UI is modern and intuitive while being secure and private. You will have to download an icon pack for having icons instead of letters for your differents icons. The app explain you how to do it !
A lightweight FOSS Android launcher forked from AOSP's Launcher3, using yuchuangu85's repo as a base.
podium is a modern, open-source Podcast app for Android, written in Kotlin using Compose. The app uses Material 3 Expressive design to fit right in with modern Android UI.
An open-source Android app built around flexible workspaces and customizable spaces—enabling you to organize notes, journals, tasks, events, and more, while adapting to your own systems as it evolves.
Traffic Light is an open-source tool for tracking your network usage while ensuring total privacy. Inspired by Internet Speed Meter and GlassWire.
Peri is period tracker application. The application has the necessary features that help you track your cycle. The app shows:
Whether you're maintaining sobriety, breaking harmful habits, or building positive routines, Grove helps you stay focused on consistent progress. Watch your streaks grow, track meaningful milestones, and cultivate a forest of procedurally generated fractal trees that evolve...
Premium audiophile grade offline music player with native DSP, Visualizer, 10-Band Equalizer with PreAmp and Milkdrop support, currently in initial alpha testing.
Open source file utility for Android with built-in media players, file transfer over Wi-Fi, storage analyzer, image grouping, and no advertisements.
The new MaterialYou version of AntennaPod is just gorgeous !
A really good open source podcast app !
Athena is a Material You (Material 3) firewall and ad blocker that works seamlessly on both rooted and non-rooted devices.
Streams music from YouTube Music on Android, supports playlist creation, background playback, song search, lyric display, favorites, and downloads.
Download app from 
Google Play Store without a Google Account. Or sign in with your Google account for downloading your paid apps on a degoogled phone
A cup of coffee is not the same anymore then back in the 80's, thats why I wanted to build a tool for the coffee community.
One of the best web browser for Android. Block ads and trackers by default and as good privacy default configuration ! I do not use the BAT cryptocrap stuff.
If you want to learn more on how to buy, store and use crypto the right way to empower your sovereignty and/or privacy look at my other list Doing crypto the right way !
A great looking multi-cryptocurrencies wallet with some privacy feature and support for 
Monero , 
Bitcoin , 
Ethereum , 
Litecoin and Haven (XHV) + the stable coins Haven protocol supports and NANO.
Note : if you want a XMR only version of this wallet look at 
Monero.com it's also from Cake Labs devs.
Chronal is a metronome and an instrument tuner app, which supports various advanced features.
Unlike other apps that use slow, bulky, old versions of FFmpeg with software encoding, Compressor uses Android's native Media3 Hardware Acceleration to process video faster than any other app.
The best looking open source and private menstruation tracking app available.
Note : F-Droid package is outdated. If you know how to make one join their GitLab !
Open Source doesn't mean ugly
Comments
Thanks for sharing! Will check some of these out!
I'd also add Element(.io) it's pretty much Signal privacy with Discord type of features and it's decentralized. I think it has better security than Signal.
edit: Ended up leaving Buttercup for KeepassXC so nvm that
But nice list! Saw some apps that i haven't heard of so I'll look into those.
You right I've forgot Element. It's not better than Signal tho, it's a different usecase. Never heard of Buttercup, will look into it.
Happy to hear that you discover new apps, I hope you'll like using them !