She was definitely RJ's best villain, IMO. She was also his funniest to read about, and the one with whom he actually had fun turning the cliché of the hedonist barbie/bimbo on its head. His others were most often the clichés as is, more or less. It's really no wonder that by the mid-series once she's entered the story, Jordan suddenly decided to give the Forsaken some real scenes (and almost all of them have been either Graendal POV or POV of another with Graendal. He would not have admitted that, but I think she was probably his favourite Forsaken to have around. I suspect Graendal was the closest in personality to aspects of his own, that is her charades and often playful mind games are the same sort RJ himself liked to play with the readers in the books. He didn't mind her having POV, her way of thinking left him plenty of room to leave a lot unsaid, whereas with the more simple(-plistic) plots of the others, he usually shoved the issue under the carpet by never explaining them and making them appear puzzling/mysterious.

That made her the most satisfying Forsaken to read about, IMO.