There are a few things to look for when determining the highest quality goose down. The first is fill power, which is a measure of how much space one ounce of down will take up. The higher the fill power, the better the quality. Another thing to look for is loft, or how well the down traps heat. Again, the higher the loft, the better the quality. Finally, you want to make sure that the down has been properly cleaned and processed so that it's free of debris and allergens.
The best goose down will have a high fill power (800+), good loft, and be clean and allergen-free. This type of down will be expensive, but it's worth it if you want your sleeping bag or comforter to be as warm and comfortable as possible.