Kingly, basically the lower the amount the better but I do know what you mean about what is best.
I follow these rules
Diary 2g per 100g or less, if there is no choice, look at what there is and get the lowest fat of all. Yogurts/Soft Cheese/Milk
Meat 5g per 100g for Beef/Chicken/Ham products
Mince 10g per 100g (although I do end up buying Aldi and its not 10g, but it is the best so far I can afford)
I also check out the carbs and if it seems high but the fat is low I will usually avoid it because I know we are supposed to be avoiding the carbs.