Depends if you mean idbase or not. Base theme tends to get dismissed because everyone thinks - 'ah flat old level design clothed in idbase'.
Even idbase can be used in new ways still - there's been a few idbase maps over the past year that have really shone and done something new.
Weather base or medievel, metal or elder is best is entirely subjective.
Monsters are a problem, but name me a good unmodified progs base map that uses only grunts and enforcers. Could be I'm forgetting something obvious there.
Madfox, I think they need to be rounder - a few extra verts or maybe even convert the original sprites, though I guess you've tried that.
Ricky, the black goat wants YOU!
http://upload.wikimedia.org/wikipedia/commons/8/8f/Shub-Niggurath.jpg