Linux, so even benchmarking software is near impossible unless you’re writing software which is able to leverage the specific unique features of Linux which make it more opimized.
True. I have no doubt that you could set up a linux system to calculate pi to 10 million digits (or something similar) more power efficiently than windows-based system, but that would include compiling your own kernel leaving out everything unnecesary for that particular system, shutting down a ton of daemons which is commonly run on a typical desktop and so on and waste a ton more power on testing that you could never save. And that might not even be faster, just less power hungry, but no matter what that would be far far away from any real world scenario and instead be a competition to build a hardware and software to do that very spesific thing with as little power as possible.