1. Arctic Zephyr
This is a really clean skin that has a light or dark theme. It runs smoothly and has a nice fullscreen music mode that shows the playlist currently playing.
This one is lightweight yet extremely functional and fast. It has a minimalistic elegance to it that I like. It is made by the same person that made Arctic Zephyr, jurialmunkey.
This one is actually an update to the default theme, confluence. To install it, add this repo http://hybrid-development-repo.googlecode.com/files/xbmc.repo.hybrid-1.0.0.zip