Harvest Moon: A Wonderful Life Special Edition

This is the highest possible quality rank your milk can have. In order to obtain it, you'll have to take good care of your cow(s) - talk to them twice a day whenever you milk them, cuddle them once a day and, if you already own the brush (500g at Van's Shop), brush her twice a day and make sure she can eat in the morning and evening, either by fertilizing the grass in your pasture or by giving her the fodder from the food dispenser inside your barn.

After a few seasons, your cow will like you enough and you'll get S-Rank milk on a frequent basis. Don't worry if you get A-Rank milk in spring and B-Rank in summer, that's perfectly normal and no indication of how well your cow is taken care of. If you want to speed up the process, you can feed your cow good fodder that you buy from the ledger to upgrade A-Rank milk to S-Rank. I don't recommend wasting your money on good fodder though for your normal cows.