Based on Ubuntu but with a bunch of manual patches + tweaks and driver support additions (particularly Nvidia). Really solid stock UI, and it even comes with a tiling WM built in.
I've been using Pop!_OS on a System76 laptop since February, and like it quite a bit.
I use plasma desktop on all of my other systems, but left this one with the defaults.. so far I really like it, but I have a problem with gnome-shell leaking memory over time, since I tend to not shut down or log out over long periods of time. If I don't restart / log out, /usr/bin/gnome-shell eats up more and more memory (as the gdm user). I let it get up to about 7GB before I updated some firmware and rebooted the laptop.
I mean, I put 32GB of RAM in the thing in an attempt to future-proof it, but this is kind of bonkers.
Most notably, Pop OS (I'm doing the damn silly punctuation) takes all of the stuff that Ubuntu has made snap-only and builds them as debs. So it's basically Ubuntu de-snapified.