It depends on your goal.
A 1Z drop off a 1-tile wide path lined with simple, inexpensive, 1-weapon-per-trap, weapon traps (a.k.a. the "dodge-this" design) is good if you want to force your enemies to backtrack. And you might want to put more deadly traps and/or cage traps in the 1Z deep pit areas because when someone falls 1Z, they're stunned, and the trap will always hit.
To get trap-immune critters to play dodge-this, you need to replace the weapon traps with upright spears/spikes and hook them up to something on repeat (dwarf clicking a lever on repeat, or a water repeater, or a critter-based repeater).
If you're just looking for bruises, broken bones, and the like, then 3-5Z is enough.
For a sure kill, you really want 12Z. While 10Z will do it 80-90% of the time, it takes 12Z to make it a sure kill. And deeper is always better. And past 12Z, water drowning is optional.
For the really dwarfy solution:
- Have a 1x20 bridge made out of "down" stairs with nothing underneath and nothing to the sides for 12Z of depth. Place a 3Z deep chamber of water above that with hatches that can be toggled to flood every other tile of the bridge when triggered. So you'll need 8-10 hatch openings. The goal is to wash the filth off the bridge and down into the pits. Supposedly, having the water flow through "down" stairs will help in turning most of it into mist, leaving you less water down below to clean up later.
- Put shooting galleries on the outside walls of the pit area so that your marksdwarves can shoot at anyone trying to run the gauntlet. Put retracting bridges or hatches at both ends so you can trap your targets and give lots of target practice to the dwarves. A side view of this would be:
W__F.._..F__W
(W=wall, _ = floor, F=fortification tile, . = open space with a 12Z drop)