Indeed. I must agree. There have only been a very few books in my experience that are even comparable to the lush, detailed complexity of Perdido Street Station. (The few would include, incidentally, Samuel Delany's Dhalgren & Stars In My Pockets Like Grains Of Sand; Orson Scott Card's Songmaster; and Katherine Dunn's Geek Love).
And I'm only 3/4 through the book. I think that reading the rest of the Hugo nominees may be moot...if there's one that's better written than this one, it would probably damage my brain to read it.
Unless the ending is terribly disappointing, or the remaining unread book Cosmonaut Keep, is utterly mind-shattering, my vote will go to Perdido Street Station for Best Novel.