I specifically wanna be pentaborane, because it has the rare honor of a perfect score on the fire safety diamond. Highly flammable, highly toxic, highly reactive, explodes on contact with air, explodes on contact with water, and lethal amounts can absorb through your skin (if it even gets that far). It was actually considered as a possible jet or rocket fuel in the 1950s and earned the nickname "Green Dragon" because it burns green. But the US government ended up rejecting it because even its exhaust is poisonous. It is in fact so dangerous that the government's supply of it sat in storage until 2000 because that's how long it took to invent a safe disposal method - a procedure whose actual scientific name is Dragon Slayer.