Butter London's Knackered has got to be the prettiest polish that I own. It's a blue/purple duochrome with holographic glitter.
Butter London: Knackered (4 coats)
The photos does not do the polish any justice. In different lighting, you can definitely see how the color shifts. Someone even commented on how it looked green at a certain lighting. (I didn't really agree though)
Application: It was very easy to apply, but definitely very sheer. My VNL wasn't really as terrible in person as in my photo (especially my index finger); it was probably because the photos were taken directly under lighting. The color builds with each coat, but there's a limit at some point.
Color: Like I said, it's a blue/purple duochrome with fine holographic glitter. The color shift is very apparent and overall the polish reminds me of nebula nails minus all the work.