However, 'Brain' is also a strong contender. It explores the intricacies of the human brain and the medical procedures and ethics related to it. The plot is full of twists and turns, and the way Cook weaves the medical knowledge into the story makes it a very engaging read.
It's hard to pick just one as his best, but 'Coma' is often highly regarded. It was very successful and was also made into a movie. The concept of the story, with the mystery in the hospital and the danger lurking for the patients, made it a standout in his works.