Debian with KDE Plasma desktop, it's unbeatable.

That valve uses Arch is irrelevant in all honesty. Proton is not a Valve product, Valve is merely one of its users and contributors, and it is not wedded to one distro..Similarly Valves own Steam packages are not distro specifi, and there are other gaming platforms to consider which also benefit from Proton (for example you can get Gog windows games working in Linux too quite easily), as well as all the Retro gaming options.

Pick a distro you personally like. I use Mint as I like the cinnamon desktop interface and the distro is pretty much good to go from fresh install. I use Mint both as a dual install with Windows on my PC and also within VMs in Windows. I still spend a lot of time using Windows because of specific games compatibility and work related apps.

EndeavourOS seems a good choice if you do want to go the Arch route but it's only something I've played with in a VM.

If you want something gaming specific then Draugar seems like a good choice - it apparently uses Ubuntu LTS but with the mainline Kernel updates optimised for gaming. But I have no personal experience with the distro.

I also see a lot of people seem to like Pop!_OS, but again no personal experience.

I've had no issues with Mint on my setup.

Fedora. Cutting edge but works out of box. Very little change in use compared to Ubuntu.

Debian is good but very stable so no guarantee for some package updates which is useful for gaming and maybe proton.

On a related note, this is pretty useful: https://davidotek.github.io/protonup-qt/

Ymmv but since I dabble a bit with audio, I went with Ubuntu Studio. It has a real time kernel, and has been fantastic for gaming.