In the State of Vermont, we grade maple syrup on color and flavor. We are allowed to sell the top four grades (Vermont Fancy, Grade A Medium Amber, Grade A Dark Amber and Grade B) in retail sized containers. All maple syrup is the same density (thickness) but color and taste can vary a lot.
There is no "best" syrup — it is all a matter of personal taste. You should buy what you like the best. That said, here is a brief description of each grade to help you decide in case you haven't tried them all.
Vermont Fancy: This is the lightest in color. The flavor is also very light and delicate. I would describe this as more sweet than mapley in taste.
Grade A Medium Amber: This is a little darker in color than Fancy. The maple flavor is also a bit more pronounced.
Grade A Dark Amber: This grade has a darker color and a full bodied maple flavor. This is the grade we usually have in our fridge and is definitely our best seller!
Grade B: This is the darkest in color. It has a strong maple flavor that comes through well in food if you like to cook or bake with maple syrup. This is growing in popularity!