Note that even in fiction, some people present truly amazing ideas while having certain technical difficulties with writing. Philip K. Dick springs to mind as an author who, on the one hand, had an especially fecund imagination for original ideas. On the other hand, his ability to write dialog was absolutely awful. The plots are fascinating enough that one is willing to overlook this, though.

Terry Pratchett will get mentioned many times here, I'm sure; his facility at playing with language is basically peerless.

I also suggest Peter S. Beagle -- particularly The Last Unicorn, but also everything else he's written. I've never seen any other writer with such an impressive use of cadence in their prose.