These are the world's deadliest plant in no particular order of toxicity.

https://www.britannica.com/list/7-of-the-worlds-deadliest-plants
https://www.businessinsider.com/hogweed-wolfsbane-devils-snare-gympie-hemlock-deadly-plants-2017-7?r

1. Hemlock - Conium
Widely known for its uses in ancient Greece as a means of execution, hemlock's most famous victim was the philosopher Socrates, who suffered the effects of the plant's most potent toxin, coniine.
Ingestion of even small amounts of coniine (6—8 leaves, or an even smaller dose of the seeds or roots, usually does it) causes death by disrupting your body's neuromuscular junctions, resulting in what is known as "ascending muscular paralysis." The paralysis typically begins in a person's legs, and ascends up the body until in reaches the respiratory muscles, resulting in death.

2. Oleander - Nerium oleander
This shrub is one of the most dangerous on the list, which is bizarre on account of how widely it's used as a decorative shrub. This bush pops up in parks, schools, and back yards all over the country, especially in the southern and western regions of the US. And while it might be nice to look at, don't go chewing on any part of it — its leaves, flowers, and fruit all contain chemicals known as as cardiac glycosides, which, while therapeutic in precise doses, can put you into cardiac arrest if ingested unsystematically.

3. Monkshood/Wolfsbane/devil's Helmet - Aconitum
The poisonous properties of members of the Aconitum genus have been known for generations; several species of the plant, for example, have long been used in preparing poison-tipped arrows for purposes of hunting and warfare. In humans, accidental ingestion can be fatal; the plants contains appropriately named aconitine neurotoxins and cardiotoxins, which lead to gastrointestinal complications, motor weakness, and heart and lung paralysis.

4. White Snakeroot - Ageratina altissima
White snakeroot is so potent that just drinking the milk or eating the meat of a cow that's ingested the plant can lead to severe sickness and even death. The plant contains a toxin called tremetol, which, when ingested by humans, can lead to trembling, vomiting, and serious intestinal troubles. The illness brought on by secondary tremetol ingestion was so widespread in the early 19th century that it became known as "milk sickness," and is estimated to be responsible for thousands of deaths at the time — including that of Abraham Lincoln's mother, Nancy Hanks Lincoln.

5. Angel's Trumpet - Brugmansia
Botanists will tell you that members of the genus Brugmansia get their nickname, "Angel's Trumpet," from their large, hanging, trumpet-shaped flowers, which can measure up to 50cm long and 35cm wide at their openings. But we like to think it's named for the trumpeting angels that will herald your arrival in the afterlife. Ingestion of the plant, which contains dangerous levels of the chemicals scopolamine and atropine, is often fatal.

6. Dumbcane - Dieffenbachia
Members of the genus Dieffenbachia are made up of cells containing microscopic needle-shaped crystals of calcium oxalate called raphides, shown here at 600x magnification.
When leaves of the Dieffenbachia plant are chewed, these tiny spears of calcium oxalate typically lead to oral irritation, excessive drooling, and swelling. In rare cases, however, irritation becomes so severe that the tongue, mouth, and throat are effectively immobilized, while tissues become so swollen that the airways are blocked, leading to death.

7.Jequirity/Crab's Eye/Rosary Pea - Abrus precatorius
Abrus precatorius is native to Indonesia, where its vibrant black, red, and white beads are often used in the region's percussion instruments. But the seeds are also highly poisonous on account of their abrin content. Abrin, which is chemically similar to the ricin found in castor seeds (see below), sabotages your body's functions at the cellular level by inactivating ribosomes, the molecular machines in your body responsible for synthesizing proteins. The abrin content of just one seed is said to be fatal to humans.

8. Manchineel/Manzanilla de la muerte ["little apple of death"] - Hippomane mancinella
The Manchineel tree does not like company. If you brush up against the tree, the white sap it produces is liable to produce a violent allergic reaction on your skin. Standing beneath the tree during a rainstorm will cause any skin that comes in contact with runoff from the tree to blister.
These epidermal effects are caused by the powerful irritant phorbol, just one of the many toxins found in the tree's milky sap. Ingesting the tree's fruit, pictured here, is often fatal on account of its physostignmine content, which can lead to complications involving naseau, vomiting, diarrhea, and seizures. Basically, don't mess with the manchineel; there's a reason it's called "the little apple of death." You won't like it, and it definitely doesn't like you.