For running safenodes? Whatever is to hand that isn’t already busy! But if I had to buy something it would be a Raspberry Pi 4 because they are cheap and have Linux on it. But other things like that are available. It’s just that I know and like them.
It would be insane to buy a Mac for running safenodes and any spare one usually has a more demanding use for it.
I personally wouldn’t use Windows unless I was stuck on a desert island without anything else. But that’s a personal thing because I hate Windows and Microsoft.
It seems that safenodes run just as well on any of them. There were a couple of issues with getting the early safenode versions for Windows to work but I think they have been worked through now.
Pick any one you can and you’ll be fine.