Impractical, yeah; but I can’t help imagining the reactions of the first platoon of Allied soldiers to see that thing coming at them.
Even further away from being built than the Ratte, but…Ithacus.
A 7000 ton SSTO troop transport, capable of carrying 1200 men with equipment.
(I wuz just gonna say VX, myself.)
What about poison gases? Both sides in WWII produced copious amounts of mustard gas, sarin, etc., but never used the stuff. Or biological agents-the Russians made anthrax, and other bacteria-but never used them.
No discussion like this is complete without a mention of HMS Habbakuk, a 200,000 ton aircraft carrier made out of ice and wood pulp. How cool is that concept?
The ekranoplans that engineer_comp_geek mentioned are pretty awesome too - there’s some footage of the larger ones in action that looks like an ocean liner just sprouted wings and took to the air.
But poison gases have been used, in WW1 and more recently by the Iraqis against the Kurds. 
To my knowledge 1920’s style death rays have not been used in conflict but would have been all kinds of awesome.
WWII. America’s “Bat Bomb” project. The main reason it probably wasnt used was it wasnt ready by the time the war ended.
The idea was you took a bat and attached a small incendiary device to its leg. Once the bat was released it would fly up under the eave of a building or an attic or bridge or in a tree. The first thing it would do is chew off this attached device. The device was designed to ignite once removed.
A cannister bomb of these fella’s could probably contain thousands if not 10 thousand of em.
Once released over a target city they would disperse over a large area and probably start many fires. Way more than the local population could handle at one time most likely.
For such fire prone areas as Japan at the time, I imagine these things would have been as devastating as the atomic bombs and the carpet bombing that caused the big fire storms.
There was book written about it a decade or two back.
I put it in the great category because if it was as effective as it sounds like it might have been, it would have been both pretty destructive and pretty darn cheap. Plus the idea is just too darn quirky yet clever.
The was also an attempt to make a pigeon guided missile, Project Pigeon.
N-stoff, or chlorine trifluoride. It burns almost anythig it contacts. When it contacts human skin it hydrolizes to produce hydrofluoric acid and hydrochloric acid. And, oh yeah, it lights you on fire, too. Thankfully the Nazis never got the chance to use it.
How about the “Doomsday Machine” (in the movie "Dr. Strangelove). Now THAT was a deterrent! Cheap to build, effective, and unstoppable!
Darn, you beat me to it.
Well, don’t forget that this was an absolutely awesome thing, and there’s not reason it wouldn’t have worked very well. The only problem was that it took do long to put together that there simply was not real reason to use it by the time the could have put it into action.
Though the neat thing about the bat-bomb is that it sounds like, in principle, the bat could have survived its contribution to the war effort. Go bats!
Dwarfed by the 1500 ton Monster, but the 100 ton Panzer VIII Maus actually reached the prototype stage, and the Panzerkampfwagen E-100 almost did.
Re: Bat Bombs: the basic concept worked pretty well for Harald Hardrada.
Here you go - but ironically, not fully automatic and explicitly NOT used as a deterrent (at least, not a deterrent for the enemy).
Not Great, but Awesomely Bizarre (and amazing it ever got the go-ahead) – Panjandrum
It was propelled by rockets along the edges of the giant double wheel, not by a single backward-pointing rocket motor. It was supposed to use this tangential propulsion to roll it up to the supposed Atlantic Wall, then blow up there. It had a dismal track record in testing
Other wonderful, failed projects:
There was also another Allied project that was supposed to send a huge flame up to deter airplanes. In practice this would have been as effective as a candle flame is in burning you as you pass your finger rapidly through it, but it was thought it might be psychologically deterring. Nah.
Hedgehog was used and was pretty successful - your own Wiki link shows it in action!
I remember seeing some video of the Soviet version of Hajile being used to airdrop an APC with crew inside. Brave men.
There’s some old film of the Panjandrum tests - it’s a pretty spectacular failure as the thing goes spinning about shooting rocket flames in every direction like a severely unbalanced clothes dryer full of fireworks.
To the OP - were you looking for the most devastating weapon developed but never actually used? Or the most unusual ideas for a weapon system? What about things that never made it off the drawing board at all?
Dr. Strangelove’s Doomsday Machineis real (and still operational).
Ha! Love it.
I’ll take your Bat Bomb and raise you a Soviet Anti-Tank Dog. Had some drawbacks, apparently…
Oops! :eek: Get away! Bad doggy! Boom 
I’m going to go for two real ships. La Gloire and HMS Warrior. La Gloire was the first ocean going Ironclad warship. It effectively made wooden hulled warships obsolete. The next year, the British launched the Warrior, the first iron hulled warship. That consigned ironclads to the scrapheap and led directly to concept of the battleship.
Neither ship ever saw combat, and were only operational for a very limited time, but between them they completely changed naval warfare.