Atom for atom, which element is the most dangerous for a human?

Mercury has two faces. As a metal it is not particularly dangerous even if you swallow it. However, lost into the environment, it gets divided into finer and finer droplets, then metabolized into methylmercury by bacteria. This is one of the most hideously dangerous toxins known.

It is a biologically cumulative neurotoxin. Once it is in an animal or human it cannot be excreted. It concentrates up the food chain, so bigger fish that eat little fish become more contaminated … and that's why eating tuna more than twice a week is no longer recommended. Even the remotest ocean now has significant mercury pollution, from human effluents and coal fired power station chimneys.

A tiny drop of the pure chemical goes straight through latex gloves, straight through almost any protective clothing, straight through skin, and causes a hideous death over the next few months. In contrast Polonium or Plutonium is dangerous only if you swallow or inhale it. Accumulating a toxic level through polluted food leads to an even more protracted fate.