One of the problems is that they have killed many characters only to later bring them back. Very few deaths seem permanent.
I still rank Supergirl's death as one of the best because it was heartfelt and meaningful. It also had a nice symbolism to it with the death of her innocence more mature comic themes followed like watchmen and dark knight.
I don't think Barry Allen's death was nearly as well done as Kara's-although it was done well.
