Arch. I've been running it on my laptop for the last 3 years, and I've only switched my gaming PC from Windows in the last couple of months. Really impressed with how much Proton has improved since the Steam Deck has come out.
Moved from Ubuntu as I was having issues with the WiFi drivers on my laptop, and both my systems have been rock solid ever since.
Still waiting for the Deck to become officially available in Australia, but I'm pleased to see the progress recently in Linux gaming in general.