The most healthful cereals are made with whole grains and not much else. If you're trying to lose weight, control cholesterol or diabetes, or just need a lot of energy, your best bet is a hot cooked cereal of whole grains, such as oatmeal; or barley, brown rice or wheat berries cooked and served like oatmeal. Flavor it with raisins or other dried fruits, cinnamon, and perhaps a handful of nuts such as pinenuts.
If you prefer cold cereal, you need to check the list of ingredients carefully. The FIRST ingredient should be a whole grain. Then scan through the entire list and if you see the words "partially hydrogenated," put the box back on the shelf. We recommend that you avoid foods with partially hydrogenated oils (or "trans fats"), and they still show up in a few cereals.

Once you've eliminated all the brands made with refined grains or partially hydrogenated oils, check for ADDED sugars (you want little or none) and fiber (you want a lot.)

Raisins or other dried fruits will add a lot of grams of sugar to the listing on the nutrition panel; they are not distinguished from added sugars, so you can only estimate the amounts. Check the list of ingredients instead.

The fiber content listed on the nutrition label can be confusing because it's based on serving size, and very light cereals (such as puffed wheat) show little fiber per serving, but an acceptable amount when you adjust for weight. Cereals made from bran (the outer covering removed from whole grains) will have higher fiber content than cereals made from whole grains (which have the germ and starchy parts of the grains as well as the fiber), but they can be hard to digest.

Update on Partially Hydrogenated Oils (Trans Fats):Partially hydrogenated oils have been taken out of most cereals. The disappearance of PHO's from cereal shelves shows that consumer pressure CAN make a difference. Still, read through the list of ingredients to make sure.

Is it really whole grain? Manufacturers have also responded to the call for more whole grains in our diet, so you will find a lot more choices that meet my recommendation of "whole grains as the first ingredient". However, many that claim to be "whole grain" still include refined grains. You may need to do some detective work to see what you're getting. One-ingredient whole grain cereals (i.e., shredded wheat, puffed wheat, oatmeal) are sure bets. If you see milled corn, corn meal, wheat flour or rice in the list of ingredients, you're getting a mixture of whole and refined grains.

Recommended: Cereals made with all or mostly Whole Grains (No trans fats, little or no added sugars; but check the list of ingredients -- recipes can change.)