Pure maple syrup is graded according to Federal USDA regulations, and is based on both color and flavor. The grades are: US Grade A Light Amber, US Grade A Medium Amber, US Grade A Dark Amber, and US Grade B. Some states use a slightly different terminology, as does Canada, but the legal requirements for each grade are the same, regardless of what they are called. For example: Grade A Light Amber syrup is sometimes called Fancy Grade, and in Canada it is called No. 1 Extra Light.
Grade A Light Amber, is very light and has a mild, more delicate maple flavor. It is usually made earlier in the season when the weather is colder. This is the best grade for making maple candy and maple cream.
Grade A Medium Amber, is a bit darker, and has a bit more maple flavor. It is the most popular grade of table syrup, and is usually made after the sugaring season begins to warm, about mid-season.
Grade A Dark Amber, is darker yet, with a stronger maple flavor. It is usually made later in the season as the days get longer and warmer.
Grade B, sometimes called Cooking Syrup, is made late in the season, and is very dark, with a very strong maple flavor, as well as some caramel flavor. Although many people use this for table syrup; because of its strong flavor, it's often used for cooking, baking, and flavoring in special foods.
All maple syrup grades are better than the artificial stuff. Otherwise it's strictly a matter of personal choice. Ask yourself (or someone else) these questions: Which is better, white wine or red wine? Which is better, light beer or dark beer? Beer can probably be compared most easily to the different maple syrup grades/flavors. A light Pilsner beer has a light color and delicate flavor, while a Stout or Porter has a very dark color and strong flavor. It's strictly a matter of personal choice, and there isn't one grade of maple syrup that is "better" than another.
Maple producers have no control over which grade they make. As a rule of thumb, lighter syrup is made earlier in the season, and darker syrup is made later. But since we are dealing with Mother Nature in our business, anything can happen. Producers have seen years where 95% of the annual crop was light amber syrup, and some years yield almost no light syrup at all, when most of the crop is dark syrup. During the six-week maple production season, the weather goes from cold to warm as spring pushes aside the cold of winter. Additionally, the trees themselves undergo metabolic and chemical changes as they go from winter dormancy to springtime activity. The tree buds start to form towards the end of the sugaring season, about a month before they open up into small leaves. These changes cause differences in maple syrup flavor as the season progresses. Experiment with the different grades, and continue to buy what you like the best. Remember: There is nothing better than pure maple syrup.
The Federal Register entry that relates to the USDA's grading standards can be found here.