I've used the same basic building block for my forts for a long time now, it just speaks to me somehow. It started as a big square that allowed easy access to 5x5 rooms in each cardinal and diagonal direction that can be adjusted to house anything, and formed the basis for a tower in the first fortress it appeared in (
http://mkv25.net/dfma/map-5354-weaverwalls), it's become something I call an 8-tuple or octuple core for lack of a better word. (edit: Skip to the end and check the other mkv25 link if you'd rather see than read)
I make this first building block by first designating an 11x11 area, carving 5x5's into each corner (2 in from the edges), then a 5x5 at each cardinal direction (1 in from the edge). The center is often dug as a 5x5 room containing a stairwell or something else, and outside that room going in each cardinal direction is a 3x3 hallway that extends through every fortress space.
Lately I have had a fortress-wise pattern that has worked well for me too, which rather mirrors the inner pattern. I build more of the 8-tuple rooms around the central core one, with a 3 or 5-wide hallway (depends whether it's between cardinal or diagonal 5x5 rooms) dividing them and providing access between the cores. In my last three fortresses, I've had on my main fortress level 9 of the octuple blocks or cores, in a 3x3 pattern with the hallways between them.
Horizontally, each core only has access to each other core on the main fortress level. Cores above and below are storerooms and/or kitchens only that must be accessed by stairway from the matching room (workshops to storerooms, dining room to kitchen & kitchen storage, living quarters to living quarters). These extra spaces (with the exception of living quarters) have outer corridors dug as well, but instead of joining to the other rooms they're just for extra storage.
Workshop spaces will see the cardinal 5x5 rooms assigned doors to seal them off for moods gone wrong, while corners have non-moodable workshops, stockpiles, or minor stairwells. Kitchen spaces are just like workshop spaces, except they may have dining quarters and food stockpiles as a dedicated layer underneath them. Living quarters see most of the 5x5 rooms converted into three, 3x1 or 4x1 shafts that become bedrooms, with the inner-most diagonal 5x5 room turned into an entrance and stairwell that allows easy access to the rest of the fort-- it's the inner-most diagonal because my living quarter cores are situated on the outer diagonals (NE, SE, etc) vs the rest of the fortress. The center space is usually for stockpiles, but in the LQ's case I make more bedrooms in the center.
Main stairways are always contained in 5x5 rooms centrally located, with doors and/or linked drawbridges to close off access to the upper or lower levels (depending on whether it is above or below the core of my fortress). Mining and cavern access isn't central to the fort, but instead is from a more fortified room leading down from the smithing / smelting block, which is itself usually located south of the center. Minor stairwells are used to access stockpiles or other additional space, and tend to be in the corner rooms.
Above my main fortress level are a couple more features I've worked on. Firstly, there will be a small surface fort on the surface, with the trade depot in the first room proper (with a roof and all) that is itself inside of a larger curtain wall. The barracks will be just past the trade depot-- my entire military is used to the outdoors without getting rained on. Also in the surface fort will be the beginnings of a refuse pile (continuing further down via artificial floors that allow airflow). Well-water access and the beginnings of plumbing will also start on this level.
Below, between the surface fort and the main fortress, on one of the soil or sand layers ideally, there will be a large excavated space that will be allowed to grow into an underground forest.
Throughout the fortress, I try to ensure wherever possible that there are sufficient chokepoints for the placement of doors.
Noble quarters, jails, and hospital spaces will be just outside of the main cores in an outer network of 3-wide or 5-wide corridors. Since the outer corridors don't go anywhere on my man fort levels, it's perfectly fine to just turn them into rooms since my dwarves won't have to walk through them to get elsewhere. In fact the outer corridors aren't even linked to eachother.
I'll post my latest finished fort that has this design up on mk25 in a minute to make it easier to read. (Here it is:
http://mkv25.net/dfma/map-10907-ironhelm)
