More deadly and deadliest are the usual forms

You can also use most deadly.

(Source: Is 2014 the deadliest year for flights?)
deadly (a)
1. causing or likely to cause deadth = lethal
a deadly weapon (Vũ khí hủy diệt)/disease
deadly poison
The corba is one of the world's deadliest snakes.
The terrrorists have chosen to play a deadly game with the civilian population.
2. extreme; complete
I'm in deadly earnest.
We sat in deadly silence.
They are deadly enemies.
3. extremely effective, so that no defence is possible.
His aim is deadly (= so accurate that he can kill easily)
It was the deadly striker's 11th goal of the season.
4. (informal) very boring
The lecture was absolutely deadly.