I have often read that plutonium is the most deadly element known to man. This is not true. I'd like to address two separate issues in answering your question. The first is the toxicity of plutonium as a heavy metal. The second is the hazards associated with the radiations emitted by plutonium and the other isotopes present with plutonium.
Plutonium is a heavy metal and as such is toxic.
There are many heavy metals that are more toxic however. I'll leave the heavy metal toxicity discussion at this point. For the purposes of this discussion we'll assume that the plutonium in question is weapons grade. In this grade of plutonium the desired isotope is Pu-239. Several other isotopes are present however.