I've been working on this one for a while - the Ultimate Survival Farm!
The farm is 3 chunks by 3 chunks - so the entirety of the farm will always be loaded while working on it.
From y=70 up, it has a alternating levels of wheat, potato, and carrot. Each level has a hopper chain going down a central column - I have the carrot chests in one corner of the column, potato in another corner, ect. This way at the bottom, the output is all sorted without needing a separate sorter (faster movement).
The central column itself is bubble elevators up and down. Each level also has stairs, but this is rapid transit.
From bedrock to y=70 is my fully-automatic pumpkin and melon farms - pistons break the melon/pumpkin as they spawn, which is collected by minecart hoppers, and transported to a central collection point on the main floor. The video walks through the process of building this farm (my own design).
The reason for such a big farm is not for the produce, but to trade for emeralds - this is a secret emerald farm by including a farmer villager on the main floor. Harvesting the wheat/carrots/potatoes, re-planting, and selling off yields 10 stacks of emerald blocks per round. Since the melon/pumpkin farm isn't finished yet, I'm not sure how much that will add.
You can check out this build, and some other really cool builds, on our server here: