It would have to be one of the nerve gases like the nitrogen mustards. However did you know that hydrogen sulfide (bad egg gas) is about 100 times more deadly than hydrogen cyanide, yet we generate it in school laboratories – or at least used to in my day. The thing is, although it is 100 times more poisonous, it is a gezillion times more smelly so we get away before it reaches a lethal dose. If you stop being able to smell bad egg gas, you are in trouble as your nervous system is closing down.
I like Laurence’s answer also, but it depends if deadliest means most toxic, or the gas that kills most people? The gas that kills most people, as Laurence suggests, is an odourless gas. Carbon Monoxide is an odourles, colourless and tasteless gas which is emitted when carbon doesn’t burn fully in enough available oxygen, and it is very dangerous. Also very dangerous are the gases emitted from cigarette smoking – up to 4,000 toxins. It won’t kill you quickly, but it can be deadly.