Just watched a Secrets of the Dead epi about the Japanese I-400 sub, which got me thinking what is the Greatest, or at least in the conversation, Weapon, for whatever reason, was never used?
h bomb
Yea, assuming you mean used as in used against the enemy during wartime, as opposed to just tested, then I don’t think there’s much to debate. Definitely the hydrogen bomb.
You could argue that the hydrogen bomb has never been used against an enemy. On the other hand, if you claim that it’s use is as a deterrent then it has gotten quite a bit of use. There’s wiggle room in that one either way.
Probably more along the lines of what you were thinking, I vote for the Ekranoplan, aka the Caspian Sea Monster. They only built a few of them, and I don’t know if they ever did anything but trial runs with them.
The chicken guided missile has to rate up there somewhere too, just for its originality.
Let me rephrase the OP to broaden it a bit for fun: What Greatest Weapons of their Historical Era Were Never Used for Whatever Reasons?
The Germans in WWII planned a 1500 ton tank with 100 crew and an 800mm gun.
http://www.manuelsweb.com/neutronbomb.htm The neutron has a special antipersonel component that makes it hideous. It releases less blast and more energy and radiation to kill people but save structures.
IIRC both the 1000 ton Ratte and the 1500 ton Monster never made it off the basic drawing board and were totally impractical anyhow…not quite an Ogre or a Bolo 
Of H-bombs I’d nominate the Tsar Bomba, the 57MT weapon that the USSR actually built and tested (with one inactive stage - it was designed to yield 100MT at full power). It wasn’t terribly practical since there were only a couple of targets (major urban areas) that were large enough to justify using it and the increasing accuracy of ballistic missiles meant that there wasn’t any need to drop something that big on a hardened target.
Most horrific weapons that never got used are probably some biological weapons - I remember reading that a relatively small amount of botulism would suffice to kill everyone on the planet.
My favorite “Mad Scientist Did They Really Design That” weapon is Project Pluto, the nuclear ramjet-powered cruise missile. About the size of a locomotive it would roar along at Mach 3 a few hundred feet AGL, guiding itself to multiple targets and tossing out a 1MT bomb on each as it went by, before finally ramming itself into a final target. Unmanned and it could stay aloft for an indefinite time period. Ballistic missiles made it unnecessary and it would have been a nightmare to actually test, but they built the engines and ran them at full blast on the ground.
There was a famous thinker (maybe from Ancient Greece, maybe from the middle ages or scientific revolution) who designed a weapon that could flip ships over. Essentially it was a crane, set up inside harbour walls, from which you could lower and attach a hook to the front of an enemy ship, then lift the bow out of the water and flip it over. Can’t remember who it was though. I don’t think it ever got past blueprint stage either…
Cobalt bombs?
Or maybe the SLAM of Project Pluto? A fallout spewing nuclear drive missile that scatters multiple nuclear warheads then criss-crosses the enemy’s homeland spreading radiation is pretty nasty. It never got past the stage of testing the engine though.
EDIT: Valgard beat me to it.
Impractical or not, that Monster tank is chock full of awesome. 
That was Archimedes and the device was called Archimedes’ claw. As far as I can recall, there’s no evidence that one was ever built.
It may be a difference without a distinction at that scale, but the Ratte and the Monster might be considered self-propelled artillery rather than a tank.
Developed too late to be used in World War One, Lewisite was a blister gas that could penetrate protective gear made of rubber. It was probably the most deadly chemical agent the US developed before organophosphate nerve poisons were invented.
Does greatest mean bad-assiest? If so, I’ll go with the B-36 Peacemaker bomber - 6, count ‘em 6, of the largest piston engines ever made, in a slick pusher configuration, AND 4 jet engines to help get the monster airborne. Just watch Jimmy Stewart in Strategic Air Command to get the idea of just how bitchin’ it was. But never used in anger, only as a deterrent.
I’d say some classified contagious biological agent(s) would be the greatest weapon never used.
I have a vague memory of reading a book or watching a documentary where the cost of casualties was estimated at about 1 dollar per death for biological agents. It depends on the agent but it gives you an idea.
It’s cheaper than a hydrogen bomb and one release could take out a bigger chunk of the population than 1 H bomb.
Ballistic Missile Submarine?
Even better than that was the XB-70 Valkyrie, a six-engined Mach 3 monster that may be, and I say this entirely without shame, the sexiest airplane ever built.
The German Elektron Bomb at the end of WW1. It was a thermite and magnesium incendiary weapon that weighed only 1kg. The small size meant that German heavy bombers like the Gotha IV and V and the even bigger Zeppelin-Staaken R.VI could carry thousands of the things. In August 1918 the Germans were 36 hours away from attacking London with them when the order came through to stand down because of a fear of reprisal raids.
How about the Big Babylon gun that Iraq was trying to develop in the late '80s. 156 meters long, 1 meter bore.
Two were planned. Most of the barrel of the first one was assembled, then the designer was assassinated and governments started intercepting the pieces that hadn’t been delivered yet. After the first Gulf War, inspectors destroyed what was left.
If you haven’t been to Dayton to see it, go.
Awesome device. I can’t get all the URL to work, but if you google Achimedes’ Claw it comes up in the first or second item. At www.mcs.drexel.edu lots of other weird and wonderfuls that might qualify.