The phrase 'most untidy' is correct and can be used in written English.
You can use it to emphasize that something is very untidy and disorganized. For example: "Her bedroom is the most untidy room I've ever seen!".
Although overlooked, a brutal first chapter had been written in the most untidy history of a "Christian nation".
It is his most untidy film by far, yet I think it is also the most promising.
Models such as this highlight the way in which policy-making can be seen as a most untidy process, rather than neatly going through a series of phases [ 53].
In another – surely the most untidy – our memories.
Our little avian friends scratching out their living, making the most of the untidy parts of our gardens.
There the four — and the scheming Swede — will remain for the next few days as fireballs from outer space transform most of humanity into untidy piles of cremains.
We are offering our empathy to other parents when we say things like, "Hey, my house is untidy most of the time, too," or, "I'm also tired as hell of changing poopy diapers today," or, "Yeah, it's gonna be beer-thirty here ASAP".
When pressed about the military's inability to secure Baghdad during the looting, Rumsfeld offered one of his most famous utterances: "Freedom's untidy".
However, the variety of them is so untidy that most theatre historians despair of defining them as a genre at all.
Most of the people have an awfully untidy apartment.
Though it is full of surprising subtleties, it represents Berlioz at his most grandiose, a formidable notion in itself: both untidy and glorious.
