Generally, the top of the head is generic , but effective . The trick spots are as follows:
1) Behind the ears because your cat cant reach there even with it's hind legs
2) Under the chin because ,again, its VERY hard for your cat to itch itself there.
3) The stomach region and upper glasis because the cat's paws cant bend 360 degrees to scratch itself
4) The tail. Why the tail? Because it feel cool ;D