I love central spiral ramps. A while back I made several spiral ramp designs in Quickfort, and I've been using two of them for every fortress for a year now. My favorites are double and single helices.
Screenshots of the two ramp designs I use most often for large, but not wagon-scale, stairwells:
These ramps, created for aesthetic reasons, still maintain high efficiency. This is because a dwarf, moving right next to the center shaft, only has to move horizontally one tile per Z level crossed.
I usually put the double helix staircase in the center of a fractal bedroom design like
this one, modified so the up/down stairs are instead statues or, in particularly large versions, little waterfalls, or even café plazas with small food/drink stockpiles, tables and chairs. If I can, I rip out all the stone that doesn't form bedroom walls (dwarves ought to have some privacy, and they love the engravings) and replace it with clear or crystal glass. The single helix is for places where long, dark, claustrophobic tunnels reaching deep into the earth are more fitting.
All of my spiral ramp designs were originally based off of a giant diagram of evenly spaced radiating lines I'd created for a city-sized spiral ramp with 32 stages per revolution, all of them exactly aligned. That was for an immense pit I built a city in back in the early days of DF2010. It took several days to complete; I still have the CSVs for it, but it turns out the amount of mined space required for more than one revolution of that helix taxed my computer too much to use.
Recently though I've taken to a new design strategy. Instead of a central staircase with rooms clustered about it, i've turned everything on its side. I have a long central hallway (with a few nice turns and bends in it) with different modules attached to it, each "module" consisting of 5-10 z levels of moderately sized rooms stacked on top of each other.
This is a good idea. Suddenly I'm wondering if it's possible to make a double helix staircase wind
horizontally into a mountainside..! Or even at a 45
o angle, though I suspect that would require using a vast amount of space to capture its complexities.
Edit: Fixed image link: the URL changed. x2