I have a complicated answer for you so hold on tight.
Polonium is the most radioactive emitting the most energy.
Lawrencium and Nobelium decay faster but the half-life lasts only minutes, you are unlikely to ever run across it naturally. Half life is the amount of time it takes for a mineral to decompose to half of its mass. For example, if you had two pounds of a mineral and it radioactively decayed to a mass of one pound in ten minutes, that mineral would have a half-life of 10 minutes. Ten minutes later you would only have a half pound and so on, until it was so minute you couldn't trace it anymore. So while the last two mentioned above are very radioactive, they decay so quickly you would never find them occuring naturally. Polonium on the other hand has a one hundred and thirty-eight and a half day half-life.
Finally, scientists are working to create new elements all the time. They decay so quickly there do not occur naturally. Ununoctium, element number118 is the latest one produced. They do not exist long enough to occur naturally. So you know, every element over number 84 is radioactive.