The best ever Mac default desktop background (aka Mac wallpaper) was the blue Aqua picture that came with OS X 10.4 Tiger. It’s classy, restful and classic without feeling dated. It has enough depth to make it interesting, but is simple enough not to distract. Other Aqua-themed Mac OS X wallpapers followed, but that one remains the best. And if you want it, you can download it in beautiful 5K resolution.
But that’s not all. Over at 512 Pixels, you can download every default macOS wallpaper ever, also in 5K.