I am voting Farming because it's the most general-purpose Mastery, and having the earliest access to Golden Animal Crackers is great. I'm also assuming that you're asking because there is no clear best answer for your situation (you aren't doing a Fishing Only challenge, you aren't spending 5 days a week in the Skull Caverns, your entire Farm isn't row upon row of meticulously cultivated Trees, etc.). The Statue of Blessings you get from Farming Mastery has some great general buffs, most of which are more useful a little earlier on in the game (friendship-related, making fishing easier, luck, speed, and energy boosts) while the other masteries have benefits more specific to the skill they come from. If not Farming, I'd pick Mining. I could always use more Iridium!