Top of this list used to be particular versions of Ubuntu
Summary: GNU/Linux "market share" in Steam grew by a quarter this past year, boosted by SteamOS perhaps, but watch the trends in distributions; even the Ubuntu-based distros aren't near the top (and SteamOS moved from Debian to Arch, so expect Ubuntu to stagnate further)
If Linux becomes a lot more reliable in the future, it'll be an "hey hi" miracle. If Linux becomes a lot less reliable in the future, we'll know why and who is responsible for it.