"Go-to" and "ultimate" aren't exactly the same thing
My starting go-to defence is hatch I can forbid instantly for long access to depot and short access to outside + forbidden doors shortcut with minecart bumper to kill thieves (and prevent them from taking control of the hatch). The doors will be retaken thanks to traffic designations.
Then I proceed to add some traps to catch wildlife attacks into entrance. 3 cages - two weapons at diagonals - then perhaps another 3 cages.
Bit afterwards, I upgrade it with detour option to bait animal on chain looking out through windows. Detour containing some manner of minecarty death, like percussion.
About same time, I give caravans a corridor to map edge. Maybe get up some anti-werebeast defences, depending on embark.
Later on, I might install some sort of artifact furniture with bridge seals to get buildingdestroyers unharmed. Ideally grate, floodgate, hatch or door.
For caverns, can do at leisure, but just a wooden door bait + cage traps right after embark (prevents FBs) with sealing up fort access afterwards until a spider is caught can be quite nice.
A ladder of bait furniture + bridge seal for FB capture can work, setup properly. Or just pop down an artifact in some public place and enjoy the spectacle of FBs fighting each other.
Longer-term, I think I like getting everyone train in military half the time - just the cross-training improving their civilian results is worthy enough, but clothing industry also matters less and "hauler interrupted by FB -> FB killed harmlessly" is always neat. Plus, military is nice quick way to kill things that won't take the bait or attack your encampment, just point and kill.
PS: perhaps DF Fort mode discussion would be most appropriate, tbh, since this seems about discussing designs in multiple forts. But everything migrated over correctly as far as I can see.