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.
The default one is great but kina boring. I was using it since the GM came out on my Gold 5s, Space Grey iPad Air and on my Space Grey 6 (purchased 2 weeks ago) but today I switched to this one on my 6 and iPad Air (sold the 5s).
Looks great with the black bezeles and it's in sync with the weather outside (it's going to snow next week here in Romania. We are close to Mother Russia. Lol).