I always go for a central 3 by 3 staircase with the middle stairs replaced by grates to allow water pour through. Every floor is a square (except for maybe the bedroom-level and the catacomb-level if I start too close to the edge) and has eight doors leading to the staircase. On ground level, there is the Trade Depot and above ground farming. On z-1, I have the farms/livestock. Z-2 is for the workshops, and right underneath them are the main stockpiles. On z-4 I have a large well-/fishing-room with pools/bathroom on every side (you never know when you accidentally flood your fortress, plus I want a legendary everything) Underneath that is an enormous reservoir that taps into my moat, supplied by a brook. Then you'll find the dining rooms, for the common folk and noblemen alike. Then the offices, and underneath them there are the bedrooms located. Fully smoothed and engraved for the nobles, rough for the commoners. Underneath, the catacombs. Fully smoothed for all. And a level below is where the seats for the (round!) arena are allocated. And underneath, obviously, the arena itself. This will be a perfect example of dwarven ingenuity. Maybe I'll load up a map of it. Then we have the main animal stockpile to provide quick access to the arena. What will come underneath that, I have no idea yet, but on the lowest level, I'll have the reservoir for the water that fell all the way down. From there it will be pumped back up to the highest top of the map, from where it will make my dwarves happy again by flowing all the way down.