That is really like asking what is the best fruit or vegetable-because everyone has a different taste preference, you are going to get a zillion different answers.
What you want to look for is lower sugar, and whole grain/high fiber. Cereals like Special K are low calorie, but they are also low nutrition, and low fiber. You might want to try a few of these out:
~Bran Flakes
~Raisin Bran
~Shredded Wheat
~Kashi Go Lean Crunch
~Kashi Good Friends
~Kashi Cinna-Raisin Crunch
~Hi-Lo cereal (in health food section)
~OATMEAL!
~Frosted Mini Wheats (if you MUST have something with sugar in it)