The one I like the most

There are an endless number of miniplayers for Spotify. So far I like Silicio the most. It has the best quality album arts, minimalist interface, can set it to always stay on top of other windows, and since the latest update it can be set to be visible on all desktops/spaces.
Plus keyboard shortcuts, if I ever need to hide it.

Fuli41 about Silicio Mini Player, v1.8