What are your favorite fictional toxins?

For those who don’t understand what I am talking about, look here.
I don’t know why, but my personal, all-time-favorite one is the Fear gas (owned by Scarecrow from Batman).
It has a big impact on me ever since I first saw it. It’s the toxin that inspired many of the killing methods featuring in some of my short detective stories. Sound like I’m a sick person right? Sorry :(
Do you have any favorite fictional toxins?
So sorry for such a sick question :(