Kaorti would be my top choice as best, most interesting new villains from that version of the book. I've always liked the Far Realms. I think the more I read, the more I like.
The Ethergaunt and ShadarKai are about equal in my mind. They play very different roles (in my head, no campaign right now), but are both interesting. I can see how to use both of them.
Erik, yes, James beat you on this one, but you aren't that far behind. We just need a few well done 'gaunt minis and a good adventure, and you'll pass him (unitl they do a Kaorti mini and adventure, but how do you make a Kaorti mini look all that interesting?).
One advantage the Kaorti have is all the related monsters, that aren't Kaorti themselves (unlike the "related" creatures also being 'gaunts).