Serena is moving to @you might have issues either way so "best" is not really objective
I would not recommend flatpack generally, there appears to be a pacman method, I would default to that
https://linuxhint.com/install_steam_arch_linux/
@starcross
How to Install and Use Steam on Arch Linux
Steam is one of the finest platforms for gamers. With the help of Steam, you’re free to enjoy the best games right away! All you need is just tinkering with the system and setting the tools properly. How to install and get started using Steam on Arch Linux is shown.