Windows 95 SP2
When this OS was finished and Bill Gates tested it he decided it was too good. It was the only operating system made entirely by Bill Gates himself. Damn, this won't irritate people, they'll like it! Because of this failure, which was achieved for the first time in windows history, the new OS wasn't published. This is why no one has ever heard of this OS before. This is the current operating system that Bill Gates uses.
There is however a secret society of pudding which obtained an illegal copy of the unbreakable OS. The pudding people published the OS in the year 2008 as Windows POOP but nobody ever noticed it because they were too busy with the attack of the 500 foot Jesus.
The pudding people left the planet shortly afterwards and because of this nobody knows what happened to Windows 95 SP2. Some people think it may have mutated into the monster we know as Cartoon Network. Others believe it never even existed in the first place. Eventually people got bored thinking of theories about it and today nobody cares anymore.
One of the most popular theories is that the OS in fact existed. After all, even a company that is so proficient at creating such unnaturally instable software must've screwed up at least once and made something actually worth buying. Other theories include that this version was actually made by aliens, that it was just an LSD-induced illusion, and that it was in fact just a Microsoft marketing stunt to make people believe that their next windows version would actually make a computer usable