This periodic table summarises daily upper intake limits for various elements (data taken from a variety of sources). Of the elements highlighted, Thallium is the most toxic. However, some of these elements, like copper, are essential minerals for which there is also a recommended daily intake. Remember, the dose makes the poison.
Some people still seem to think they are, however. Pre-scientific forms of medicine, such as Ayurveda, are still based on the elements being Earth, Air, Water, and Fire. A number of more modern forms of alternative medicine also take this ancient system as a basis. And what about splitting the beer atom?