Probably the 210mm Iraqi Al-Fao howitzer, arguably the most effective and formidable self-propelled gun ever built.
It was very, very sophisticated for its time. If Jane's is to believed, this thing was more advanced than any piece of field artillery the US or Soviet Union had in service. Naturally the Iraqis couldn't have come up with this themselves, they hired one of the world's premier artillery scientists and a crew of foreign technicians to carry out all the design work. The technology for the gun was also stolen from several countries.
The Al-Fao never went into production, probably because of its astronomical projected production costs.
There was only one prototype, but after being exhibited at a few arms shows it was never seen again. A smaller version of the same SPG called the Al-Majnoon (a prototype which went on display with the Al-Fao and was likewise never seen again) was found in an underground bunker in Iraq after the 2003 invasion, where it had been parked and then forgotten. The US forces which discovered it didn't say what they had in mind, but I'm guessing it was scrapped.
The Al-Fao remains missing, and what happened to the single prototype is a mystery for the ages.