Answering the OP's question of which iOS 8 wallpaper is the prettiest, I quite like the white mountain range one on my white iPad.
I have to say though, I don't really like any others. They're either too dark (what is up with the almost-black sea, moon and earth wallpapers?) The flower ones are a bit weird, and the default Milky Way one is OK but a tad dark and boring.
Where are the punchy, vibrant iOS 7-style wallpapers? I still much prefer the colourful geometric wallpapers, and the space and nature ones to any new iOS 8 one.