Walter Martin wrote some great books on the cults.
Walter Martin was a treasure. A man who knew the Bible inside and out. His Book on the Cults influenced me to leave Mormonism.
Hank Hannegraaf, IMHO, was a fraud. A man whose theology was fluid and whose understanding of the Bible was dwarfed by most of the callers to his show.
His hostile takeover of the Christian Research Institute was a disgrace to the memory of Walter Martin. The Orthodox can have him. I think eventually they'll end up wishing they hadn't laid hands on him.