So it depends on what you do with your phone and what specifically you mean by sovereignty.
If you can away with it, go with a "dumb" phone / feature phone. Some run KaiOS, some are minimalist and run their own operating systems. Some run a stripped down version of Android. The android ones spy on you a lot.
The two big smart phone OS's are Android and iOS, of course. Both spy on you.
A decent option is going with a fork of the Android Open Source Project (AOSP) or "degooglefied" android. Something like GrapheneOS or LineageOS or eOS are great places to look. They have a lot of support and community.
Some you can get preinstalled, like eOS on Murena phones. Others you install on aftermarket phones. Look for "supported" phones to get examples of this.
There's also Linux Phones.
Purism's Librem 5 is great and you can buy it preinstalled with linux and they have great support.
If you want to install a Linux mobile OS on an aftermarket phone, you can go with PostmarketOS or Ubuntu Touch (ubports).
There's also SailfishOS.
Your mileage may vary, so dive in and have fun.