Non-figurative ones are boring, although beautiful: we had our share of those during previous editions of Plasma. But maybe they have a more corporate vibe... And they definitely are in line with current Plasma identity.
On the other hand, middle right and bottom right are really nice. I think I like middle right in particular, but it's probably too cute for me to be seen having such a wallpaper;). Bottom right it's much safer!
(That said... I will just switch to one of the "picture of the day" plugins, as usual).