Original Xbox, probably. 360 emulation is still pretty rough. I doubt anyone has a functioning One emulator and definitely not a Series X emulator. Not much interest since almost all of it is on Windows anyway. The only reason I’ve been watching 360 emulation is for Fable 2.
Also, it’s fairly unlikely that Valve would include an optical drive unless they want to license blu ray stuff from Sony.
A new take on the Steam Machine could potentially knock Xbox out of the market in their current state, and I’m okay with that.