I've always preferred older women, so the hottest for me would be Missandre, and that Sands woman who came with Oberyn. I love her subtle acting because although she doesn't have that many lines, she makes sure her character's presence is felt even when the focus is on those around her. An example is when Tyrion questioned Oberyn about armor and drinking before the fight, she turns her head and looks at him as if to say "what on earth would you know about that little man" and twirls her body seductively, without having to say a word.