Belgian greyhound combs are the best to use with longhaired cats, although they have become very hard to find. Sometimes vendors at cat shows carry them. Peak Pro Tech combs can be ordered from veterinary catalogs and are comparable to the greyhound comb. The best size to use with longhaired cats and cats with dense coats is a 7.5-inch-by-one-inch comb that has both coarse and fine teeth. Combs that are 4.5 inches by one inch are good for all breeds. Those come in "fine/fine," "coarse/coarse" and "coarse/fine" teeth.
For more information in grooming your kitty, go to Grooming Your Cat.