Aconite

"Well, I think I should take some of this with me from now on. You never know when you might need to kill someone."

- Merlin to Gaius

Aconite (also called Wolfsbane) is the deadliest poison known to man.

History
Merlin used aconite to try to kill Arthur while under the influence of a Fomorroh. He used it to poison Arthur's lunch, only to discover afterwards that Gwen had already prepared him something.

Furious, Merlin threw the poisoned food into a pigpen. It was eaten by one of the pigs, who died. When Gwen discovered this, she realized what Merlin had tried to do and ran to tell Gaius about his strange behaviour (A Servant of Two Masters).