3 and 4 are nice but as something someone would set themself. Too much character and detail to be the default when Plasma do not target any specific demographic.
1, 2 and 5 are nice abstract wallpapers, but honestly boring as we have stuff like that for years.
6 is the best. This is wallpaper with some style, but not too much character.
Edit: Just in my opinion and for my eye of course.