Ubuntu Touch is by far the best OS I have tried on the PinePhone. I am really impressed. Very responsive, no major UI issues of windows not fitting within the screen size, gestures work very well. Briefly tried the OpenStore to install some web apps, haven't tried any flatpak or anything yet.
@supernova that's pretty neat. I'm thinking of switching to mint on my desktop.
@Wetrix That is an excellent choice. I recently switched to that from Debian on one of my hone PCs.