I liked Tiger, Snow Leopard, Mountain Lion, Yosemite, and Mojave's default wallpapers. Hated Lion's (too much of an eyesore), Mavericks', and High Sierra's (literally looked stoned) default wallpapers.
(Note: when I am referring to "default wallpapers", I am talking about the one that shows up when you install or boot the respective operating system -- for example, Mavericks' blue ocean wave wallpaper, and Mojave's desert landscape.