Amanita phalloides (Death Cap).
This WILL kill you if ingested, even in relatively small quantities, unless you are fortunate enough to get a liver transplant very rapidly.
There is no known antidote, and only a few supportive medicines that can "Improve survivability", but not neutralise the poison.
Hemlock can be treated with supportive medicine (ventilation) until the liver can metabolise it, as can any paralytic.
I guess I'm thinking of it from an attack/defence point of view (Security is my trade), and from that perspective I'd be more worried about Atropa belladonna (Nightshade, containing atropine) or Digitalis than hemlock. Nightshade is again fairly easy to treat (if caught quickly and depending on dose), but digitalis toxicity can really screw your heart up.