The tried and true land mine i.e. anti-personel, anti-tank, bouncing betty etc. Extremely cost effective if not down right dirt cheap (pun intended), low maintenance if any and very little implementation cost. Most can be planted via cluster munitions and cover sq. km’s of real estate in seconds for example.
Being made from non-metallic substances makes detection a nightmare, if even possible to be done, and disarming is a death sentence for many of the combination/multiple trigger types. Metal based are as difficult to deal with as well just can be detected.
They can be made in a garage, warehouse, backyard or bedroom using chemicals found around the house. Standard munitions or unexploded ordnance can be converted quickly into a mine.
Trigger can be remote, pressure, motion, thermal, vibration, trip wire etc….possibilities are only limited by imagination.
So 1 soldier or one cluster munition could deny an area hundreds of meters in size or greater with a set and forget munition that cost pennies with very little implementation costs. Using a soldier or soldiers, takes cost to near nothing compared to conventional/biological/chemical or nuclear weapons delivery.
Mine fileds can be laid fast, diverting enemy units into presighted kill zones or used for area denial. Germany liked to do this during fighting withdrawls in WW2.
The psychological effects on both military and civilian populations can only be met by nuclear or biological/chemical weapons. I would have greater fear for a land mine then any other.
Downside of course is its a non biased killer so if the field is poorly recorded, marked or cleared at the end of hostilities, it could bite you back. Africa, Middle East and Southeast Asia are still heavily mined decades after thier wars ended. Added this due to excellent comment: they also can be found, moved and reused against you by your enemy. Also ground movement/errosion is another factor.
Nothing else compares except maybe a long life biological weapon or a salted nuclear device but from a cost vs. effect point of view, the land mine wins hands down. Also people were so bored with taking limbs off, they developed nuclear land mines for some extra fun.
There is no other munition on Earth that carries a better resume then the land mine.
Ease of use
Training of soldiers to install and set live…kids could do it within minutes
The enormous amount of configurations and variations to fit all situations needed (personnel, shipping, armored vehicle etc).
Long service life…WW2 mines still go off in 2017.
Most just disable vs. kill the unlucky fella, so you are removing combatants (takes 2 to carry 1) from the field and consuming enemy resources treating injured behind the lines.
Now name a munition that can cover all of those points because i can think of none others at the moment. I would call it the Swiss army knife of killing people.
By no means am i trying to glorify this or any munition. Mines are a horrible invention of man and a faceless, random killer. Even with the current mine which disables after X time, they should all be banned from existence.
If only humanity put as much thought into making the world better as it does killing each other, it would be a wonderful life for all.