Not the most dangerous of substances, (though, please don't get me wrong - it's plenty dangerous) but I wanted to pick one which wouldn't kill the dragon in the process of exhaling it. It's a simple liquid made up of N2H4, but reacts very violently when catalyzed, typically using a sheet made of iridium and aluminum oxide, forming hydrogen gas (flammable), nitrogen gas (non-flammable), and ammonia (flammable). The reaction is exothermic to the point where it apparently results in a tempature of 800 C in milliseconds. It creates a vast amount of hot gas from a small amount of liquid, and quite unsurprisingly, it's used as rocket fuel.
The other nice thing about it is that it can be synthesized from ammonia and hydrogen peroxide. And ammonia products happen to be a waste product of most life on earth, meaning it's plentiful. Hydrogen peroxide can be synthesized from quinnones using oxygen and hydrogen - the anthraquinone process. No rare elements like Fluorine that the dragon needs to get - this is honestly a natural by-product. The catalyst means that the dragon can choose to only turn it into fire upon hitting the air - maybe using it's teeth to host the catalyst, or something of that nature. It does happen to be fairly toxic, a likely carcinogenic, and thought to cause birth defects. Also, if ingested, symptoms include, but are not limited to: cough, burning sensation, headache, confusion, drowsiness, nausea, shortness of breath, convulsions, unconsciousness, burns in mouth and throat, abdominal pain, diarrhoea, vomiting, shock or collapse. But it's a dragon, so it'll probably be fine.