Extra Celestial
A nicely constructed solo level, looking like a well-lit space station. As far as I can tell, there's only one suicide pit (and there's a save term just before it)... otherwise, this is a phenomenally well-designed map. Even though there's no plot (besides kill everything), and getting to the end only teleports you back to the beginning, this one is really satisfying.
 
           Extra Celestial
 Extra Celestial