Kubuntu, because at the time that was the only apt-based distro that both Steam and the proprietary AMD graphics drivers officially supported. (I had to use proprietary drivers because I bought a Vega 56 on release day and open-source ones weren't immediately available.)
Last time I had Windows was in 2011, yet this year and the last through some random fate I've had some Windows on my charts, this year it was work related, last year it was my wife's PC plugged to the TV for playing some couch games that were running sluggish on the deck.