In terms of UI, UBPorts with lomiri seems to be the best, however it's extremely restrictive in what you can do with it. (Read only root, fstab resets itself after every reboot). It's also based on Ubuntu 16.04.
Personally I have Mobian on it, which is basically Debian with tweaks to work on the Pinephone. You have access to everything in the same way you would have on the desktop. It's using Phosh for the interface.
PostmarketOS is cool, but based on alpine and I have no idea how alpine works.
Fedora and Arch also have (unofficial) Pinephone spins similar to Mobian, so get whatever package manager suits you best.
There are also the Manjaro editions, but I don't really like Manjaro.
That's the stuff I tested, though there's more on the Pinephone wiki.
