Here's a list of the most toxic compounds found in plants. (NB this does not include fungal, animal or synthetic poisons or toxins).
  1. Ricin this glycoprotein comes from the castor bean, and in mice has a LD50 of only 0.11, intraperitoneal.
  2. Nicotine this alkaloid (which I'm sure many of you know and love) has a LD50 of 300, intravenous. There's enough nicotine in a cigar to actually kill a person, if injected.
  3. Strychnine an alkaloid from the nut Strychnos nux-vomica, in cats this has a LD50 of 750, oral.
  4. Cymarine a compound of the digitalis glycoside type, found in the Strophantus variety of a tropical creeping shrub. LD50 (rat) of 25,000, intravenous.
  5. Tubocurarine chloride (curare) another alkaloid found in the Chondrodendron tomentsosum plant, LD50 of 33,200, oral.
  6. Atropine found in deadly nightshade Atropa belladonna, LD50 of 400,000, oral. This alkaloid atropine is also used to counter the effects of nerve gas or mushroom poisoning.
Notes. 1. The lethal dose figures given are all in units of micrograms per kilogram of body weight of the victim.( ug kg-1) The victim in all cases is the poor old mouse, except where noted.

These figures are taken from the book "World Records in Chemistry" by Faust, Knaus, and Siemeling ISBN 3-527-29574-7 published in 1999 by Wiley-VCH.