Because it is a naturally-occurring element that releases a huge amount of energy, many sources cite polonium as the most radioactive element. Polonium is so radioactive is glows blue, which is caused by excitation of the gas particles by radiation. A single milligram of polonium emits as many alpha particles as 5 grams of radium. It decays to release energy at the rate of 140W/g. The decay rate is to high that it can raise the temperature of a half gram sample of polonium to over 500°C and subject you to a contact gamma-ray dose rate of 0.012 Gy/h, which is more than enough radiation to kill you.
Other elements besides polonium actually emit more particles, such as nobelium and lawrencium. The half-life for these elements is measured in mere minutes! Contrast this with the half-life of polonium, which is 138.39 days.
According to the Periodic Table of Radioactivity, at this time the most radioactive element known to man is element number 118, which has the placeholder name Ununoctium. The decay rates for the latest man-made elements are so fast that it's hard to quantify how quickly they break apart, but element 118 has the heaviest known nucleus to date. These elements break apart essentially the instant they are created. It is reasonable to expect the title of "most radioactive" will be taken over by some new, as-yet-undiscovered element. Perhaps element 120, which scientists are working to produce, will be the new most radioactive element.