Great thread! Agriculture is easily my favorite industry in DF. I really enjoy balancing the inputs/outputs and the wide range of decisions/products possible (the same plant could become alcohol
or pants!)
I am always trying new things and giving myself new challenges. For example I recently tried a dimple cup and beekeeping only fort (cook the dimple spawn into biscuits/roasts as the only food source). It was a disaster but a FUN disaster!!

That being said I do have a "standard" setup that works well for me. This assumes a standard embark with 5 of each seed:
As soon as I've dug out a soil/sand room, I make a temporary farm with six 1x5 farm plots. Each plot is dedicated to one of the six crops, and lies fallow during the off-season (or grows plump helmets if I really want a big food surplus). This farm will supply plenty of food/alcohol for the first year of my fort, and a single dwarf with growing/brewing/threshing/milling can manage this farm all by himself.
By summertime, my miner(s) has dug out dining hall, work rooms, stockpiles, etc. and it's time to build a second farm for my "cash crops." This is an 11x11 room with four 5x5 plots at the corners, a statue in the center square, and seed stockpile in the remaining squares. Plants never wither because there are always idlers congregating at the statue, which also gives my farmers happy thoughts.
I often let the first migrant wave determine what my four "cash crops" will be. If a dyer arrives I grow dimple cups, if a weaver shows up I grow pig tails, if a miller arrives I grow sweet pods and cave wheat, etc. One of my cash crops is
always quarry bushes!
Here are the typical results of this plan: By autumn, I've produced enough Roasts to buy everything I need/want from the caravan (cloth and bags for my mill industry are often the first priority). Food is my only export--I don't bother with rock mugs any more since learning about quarry bushes! By year 2, roast stacks approach 10,000 in value, by year 3, they are sometimes over 30,000. Excess production is hoarded away and famine is unheard of.
My agriculture-themed forts are frequently vegetarian; I don't embark with or buy animals, and those that arrive with the migrants go in the petting zoo to give happy thoughts. I don't generally do a lot of above-ground farming (because I consider the lack of seasonality exploity) but when I do, I have a fondness for rope reeds and longland grass. The elf and human traders bring enough vegetables to provide for a nice variety in alcohol (stored in rock pots to supplement my thousands of pots of dwarven wine), and I use the seeds left over from brewing these above ground crops as training material for my culinary academy.
So that's my strategy in a nutshell. To recap:
1. Starter farm with six 1x5 plots
2. Cash-crop farm with four 5x5 plots
3. Export products/industries determined at random by arrival of skilled migrants
4. Quarry bush roasts = PROFIT