It can depend on the situation.
In everyday situation I want something that:
marks my territory clearly,
deters passive intruders,
slows or deters dedicated intruders,
gives me warning of intruders.
I'd also want it to be easily maintained and defended.
So, the obvious choice for me is a good perimeter fence, with sliding gate. Could be in the form of chain-link, thick brush, or stone/concrete and wrought-iron.
- While some may say that a fence is low-tech, requires someone to man it, and at best only slows enemies, I find it to be a good legal deterrent. And since any defense can be overcome all any defense can do is slow intruders down.
A strong fence will keep out passive intruders who look for the easiest of targets, because the fence itself is an obstacle anddepending on height and construction it may also keep out moderately determined intruders because it may require planning to overcome it
It also marks your territory clearly, so there are no "accidental" intrusions, which may help in dealing with L.E. after any altercations aswell. (No one can say they didn't know they shouldn't have been there when they climbed over a 7ft fence with barbed wire and a "do not trespass" sign!)
Its low-tech, so its pretty easy to maintain. That's a big one for me, KISS, right?
If you want to go complex: security lights, cameras, bells can be added to make a fence more effective as a warning device. All those fancy doodads are hard to maintain and could fail in a power-outage or SHTF.
I also feel that the "extras" give a false sense of security, because its easy to rely on the system to tell you tat everythings fine, instead of being vigilant.
A strong fence with a gate, maybe a dog for warning and extra deterrence, and a vigilant guy with a gun in the house watching... those are the best perimeter defenses.