Let's get this straight, the Joker hates every member of the Bat family, besides his darling, Batman.
However, there is a special amount of hate for Dick Grayson. He mentions it quite a few times in the comics, once stating that he hates him most of all. It is implied that the Joker hates Dick because Dick was the only Robin whom he considered both an outsider and the one he never managed to get to.
While every other Robin, especially Jason, was pretty messed up by the Joker at some point or another, and were quite traumatized by the clown, Dick always kept his bright and happy personality no matter what the Clown Prince of Crime threw at him.
Official license One size fits most Adjustable back Material: acrylic, wool & polyester USA import