For me it is Slaanesh, I love the idea of letting nobles (easiest beings in t he world to corrupt) do your dirty work for you, while you get to just sit back and seduce your enemies with love and parties. lol.
I played Slaanesh in Warhammer fanatsy during the 90's. I liked their looks. Slick and sexy...er well sexy for a demon i guess. The new artwork for the 6th edition demons is no very appealing to me though. I don't think I can see how this can seduce me.
If anything, maybe I would pee myself and crouch into a corner. lol