What's the most profitable thing to plant in a farm? I'd assume in most cases people plant for their profession or whatever raw material sells for the most. I, however, am a chronic alt-oholic and have every profession maxed and available to me. I don't raid and can't stand dailies, so I only have patterns available that are vendor trained, bought with spirits of harmony, or learned off a cooldown. So what's the start and end of the equation?
So what is the most profitable thing to plant on your farm? Matt assumes correctly that the majority of people simply plant what they can use to avoid having to buy off the Auction House. There is plenty of opportunity to improve this, though!
Generally, the least profitable thing to farm is vegetables. This is only true because everyone else already farms them, and they're all you can farm until you get farther into the Tillers reputation grind. The reputation seeds for leather, ore, cloth, or enchanting mats are generally lower yield than simply getting Harmonies and trading those for what you need, and that's generally less than you'd get by planting Enigma Seeds, selling everything you get, and using that gold to buy leather, ore, cloth, or enchanting mats. In short, the most profitable thing to plant is usually Songbell Seeds or Enigma Seeds.
Songbell Seeds provide you with Motes of Harmony, which provide Spirits of Harmony. If you have a profession that requires these to make items valuable to other players or allows you to use them to skip daily cooldowns, then you can figure out exactly how much one of them is worth and do the math.