The mobile world is dominated by Apple's iOS and Google's Android. Since iOS is not open source the only avenue that is left to explore is Android. And, surprisingly, there are options available that actually work very well. Which one you choose depends on the degree of data sovereignty you want and on your technical skills.
Android is an open source project, but being built by Google, it is made to function with Google's services. If you want to experience all the features and convenience of the Android ecosystem you need to have a Google account (you need one to download applications from the Play Store...). So it's important to mention upfront that any version of Android that tries to free itself from Google will result in some loss of functionality for the end user.
I set 4 criteria for the selection of the mobile operating system:
- It had to be open source
- It had to be as Google free as possible
- It had to have a descent app store
- It had to support modern phones
These are strong constraints that leave you with a very limited amount of options. Based on my research and tests it leaves you with 3 quite good options: LineageOS, LineageOS for MicroG and /e/ OS. I've used all of them for 6+ months at minimum.
All of them are Android based so constraints 1, 3 and 4 are handled. So the tie-breaker would on the level de-Googlization.
LineageOS is a fork of Android that can be installed with or without the Google Apps (incl. the Play Store), but the OS still relies on the Google APIs (localization, maps, etc...) so a lot of your data still goes to Google. LineageOS does have some nice additions on the customization and app permissions front (Privacy Guard). It is a very mature project and a very solid alternative. The other 2 options are in fact fork of LineageOS.
LineageOS for MicroG is a version of LineageOS that includes MicroG out of the box. MicroG is a re-implementation of the Google APIs that target privacy respecting services, instead of Google's. This means the OS comes without Google Apps by default. The Play Store is replaced by F-Droid that is fully integrated within the OS. You can manually install MicroG on LineageOS but LineageOS for MicroG makes it even easier and seamless for you in a nicely integrated package that really decreases the amount of data sent to Google.
/e/ OS is a project that deliberately aims to go as far as possible in the deGooglization of Android. It not only integrate MicroG. The team actually goes deep into the code of Android to remove all the possible calls to any Google server/service. On top of that they have provide their own App Store, App Launcher and their own cloud services (contacts, calendar, file sync, etc...) based on Nextcloud. This means that if you have your own Nextcloud instance, getting things set up is as easy as adding an account in the Android settings (/e/ OS is also tightly integrated with WebDav).
The Good Things
The operating system is quite pretty stable and you get (mostly) monthly updates. It's much more seldom than what LineageOS provides but I consider this to be good enough. One thing to note is that not all phone run the latest Android versions. I have a Samsung S9 which runs under Android 8 (Oreo).
The App Store and App Launcher are both average in my opinion but remain functional. For the former, not all apps are available so you can always install F-Droid or Aurora Store. The App Launcher is very much inspired by earlier iOS versions, i.e. all icons are on the home screen and no app drawer. Widgets are only available on a specific screen which means you basically seldom use them. I do think it might be over simple/basic but at the end of the day I don't really find myself needing much more.
The integration with Nextcloud is fantastic. No need to install 3rd party software since the WebDav protocol is supported out of the box. This makes is super easy to sync contacts, calendars and even files.
Lastly, one of the best features of this project is that you actually buy refurbished phones with /e/ OS preinstalled (Europe only at this time)! Samsung Galaxy S7, 8, 9+ and the Fairphone 3, 3+ are available. So if you go for /e/ OS on a Fairphone you basically have the most environmental and privacy respecting setup out there!
The Annoying Things
No Google Service means that you will have to do without some of Google's incredible applications. The one thing I can guarantee you will miss on /e/ OS is Google Maps .
There is simply no free or Open Source alternative that comes even close to Google Maps. /e/ OS ships with Magic Earth. It uses data from OpenStreetMap. The app itself is very stable, maps look great and directions are very clear.
BUT, it desperately lacks data about businesses, which significantly decreases the usefulness of such an application. Even if you do find your local store on it, you won't have links to their website nor opening hours, for example. When I need to go to a place I've never been before I usualy look it up on Google Maps on the web and then use Magic Earth to get me there. It's super annoying and time consuming. Traffic data is also not super accurate and has played tricks on me more than once.
Overall /e/ OS is a very compeling product and after 6+ of dailt use on several devices (OnePlus 5T, Samsung S7, Google Pixel and Samsung S9) I don't think I have any reason to go back to standard Android. I am starting to make my piece with the Google Maps issue...
I haven't explored Linux phones yet. They might be an alternative in the future. But currently they do not have a proper application ecosystem so I don't think they can be considered as your main mobile OS (I would be happy to be proven wrong!).